Display device having a buffer layer formed over a channel protective layer

ABSTRACT

A display device including a thin film transistor with high electric characteristics and high reliability, and a method for manufacturing the display device with high mass-productivity. In a display device including an inverted-staggered channel-stop-type thin film transistor, the inverted-staggered channel-stop-type thin film transistor includes a microcrystalline semiconductor film including a channel formation region, and an impurity region containing an impurity element of one conductivity type is selectively provided in a region which is not overlapped with source and drain electrodes, in the channel formation region of the microcrystalline semiconductor film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display device in which a thin filmtransistor is used at least in a pixel portion and a method formanufacturing the display device.

2. Description of the Related Art

In recent years, technology that is used to form a thin film transistorusing a semiconductor thin film (with a thickness of several nm toseveral hundreds nm) that is formed over a substrate having aninsulating surface has attracted attention. The thin film transistor iswidely applied to electronic devices such as ICs and electro-opticaldevices. Development of a thin film transistor particularly as aswitching element of an image display device has been accelerated.

As a switching element in an image display device, a thin filmtransistor using an amorphous semiconductor film, a thin film transistorusing a polycrystalline semiconductor film, or the like is used. As amethod for forming a polycrystalline semiconductor film, there is knowna technique in which a pulsed excimer laser beam is shaped into a linearlaser beam by an optical system and an amorphous silicon film is scannedand irradiated with the linear beam so that the amorphous silicon filmis crystallized.

Further, as a switching element in an image display device, a thin filmtransistor using a microcrystalline semiconductor film is used (seePatent Document 1: Japanese Published Patent Application No. Hei4-242724and Patent Document 2: Japanese Published Patent Application No.2005-49832).

A known conventional method for manufacturing a thin film transistor isthat an amorphous silicon film is formed over a gate insulating film; ametal film is formed thereover; and the metal film is irradiated with adiode laser beam to change the amorphous silicon film into amicrocrystalline silicon film (see, for example, Non-Patent Document 1:Toshiaki Arai et al., SID 07 DIGEST, 2007, pp. 1370-1373). According tothis method, the metal film formed over the amorphous silicon film isprovided to convert optical energy of the diode laser beam into thermalenergy and is needed to be removed in a later step to complete a thinfilm transistor. That is, the method is that an amorphous semiconductorfilm is heated only by conduction heating from a metal film to form amicrocrystalline semiconductor film.

SUMMARY OF THE INVENTION

A thin film transistor using a polycrystalline semiconductor film isadvantageous in that the mobility is higher than that of a thin filmtransistor using an amorphous semiconductor film by two or more digits,and a pixel portion and a peripheral driver circuit of a display devicecan be formed over the same substrate. However, the manufacturingprocess of the thin film transistor using a polycrystallinesemiconductor film is complex compared to the case of the thin filmtransistor using an amorphous semiconductor film due to crystallizationof a semiconductor film; accordingly, there are problems in that yieldis decreased and cost is increased.

In view of the above-described problems, it is an object of the presentinvention to propose a display device including a thin film transistorwith high electric characteristics and high reliability.

The present invention is a display device including aninverted-staggered (bottom-gate) transistor in which a channelprotective layer is provided over a microcrystalline semiconductor filmincluded in a channel formation region. A summary of the presentinvention lies in that an impurity element of one conductivity type iscontained in the channel formation region of the microcrystallinesemiconductor film which is not overlapped with a source and drainelectrodes and is overlapped with the channel protective layer, at alower concentration than in a source and drain regions.

In the present invention, a channel-stop inverted-staggered thin filmtransistor in which a microcrystalline semiconductor film is used for achannel formation region is included. In an inverted-staggered thin filmtransistor, a gate insulating film is formed over a gate electrode, anda microcrystalline semiconductor film (also called a semiamorphoussemiconductor film) which functions as a channel formation region isformed over the gate insulating film. A channel protective layer isformed in a region which is overlapped with the channel formation regionof the microcrystalline semiconductor film. Further, a buffer layer isformed between the microcrystalline semiconductor film and the channelprotective layer or over them. Pair of source and drain regions areformed over the channel protective layer and the buffer layer, and apair of source and drain electrodes which are in contact with the sourceand drain regions are formed. In the channel formation region of themicrocrystalline semiconductor film, an impurity region containing animpurity element of one conductivity type is provided selectively in aregion which is not overlapped with the source and drain electrodes.

According to one feature of a display device of the present invention, atransistor is included. The transistor includes a gate electrode, a gateinsulating film over the gate electrode, and a microcrystallinesemiconductor film including a channel formation region over the gateinsulating film. A buffer layer over the microcrystalline semiconductorfilm, and a channel protective layer in a region which is overlappedwith the channel formation region of the microcrystalline semiconductorfilm, over the buffer layer are included. A source and drain regionsover the channel protective layer and the buffer layer, and a source anddrain electrodes over the source and drain regions are included. Animpurity region containing an impurity element which imparts oneconductivity type is provided selectively in the channel formationregion of the microcrystalline semiconductor film.

According to one feature of a display device of the present invention, atransistor is included. The transistor includes a gate electrode, a gateinsulating film over the gate electrode, and a microcrystallinesemiconductor film including a channel formation region over the gateinsulating film. A channel protective layer is included in a regionwhich is overlapped with the channel formation region of themicrocrystalline semiconductor film. A buffer layer is included over themicrocrystalline semiconductor film and the channel protective layer. Asource and drain regions over the buffer layer, and a source and drainelectrodes over the source and drain regions are included. An impurityregion containing an impurity element which imparts one conductivitytype is provided selectively in the channel formation region of themicrocrystalline semiconductor film.

According to one feature of a manufacturing method of a display deviceof the present invention, a gate electrode, a gate insulating film, anda microcrystalline semiconductor film are formed. A buffer layer isformed over the microcrystalline semiconductor film, and a channelprotective layer is formed in a region which is overlapped with achannel formation region of the microcrystalline semiconductor film,over the buffer layer. A source and drain regions are formed over thechannel protective layer and the buffer layer, and a source and drainelectrodes are formed over the source and drain regions. An impurityelement which imparts one conductivity type is added selectively in thechannel formation region of the microcrystalline semiconductor film,through the buffer layer and the channel protective layer with the useof the source and drain electrodes as masks.

According to one feature of a manufacturing method of a display deviceof the present invention, a gate electrode, a gate insulating film, anda microcrystalline semiconductor film are formed. A channel protectivelayer is formed in a region which is overlapped with a channel formationregion of the microcrystalline semiconductor film, and a buffer layer isformed over the microcrystalline semiconductor film and the channelprotective layer. A source and drain regions are formed over the bufferlayer, and a source and drain electrodes are formed over the source anddrain regions. An impurity element which imparts one conductivity typeis added selectively in the channel formation region of themicrocrystalline semiconductor film, through the channel protectivelayer with the use of the source and drain electrodes as masks.

According to one feature of a manufacturing method of a display deviceof the present invention, a gate electrode, a gate insulating film, anda microcrystalline semiconductor film are formed. A channel protectivelayer is formed in a region which is overlapped with a channel formationregion of the microcrystalline semiconductor film, and a buffer layer isformed over the microcrystalline semiconductor film and the channelprotective layer. A source and drain regions are formed over the bufferlayer, and a source and drain electrodes are formed over the source anddrain regions. An impurity region is formed by selectively adding animpurity element which imparts one conductivity type in the channelformation region of the microcrystalline semiconductor film, through thechannel protective layer with the use of the source and drain electrodesas masks. The impurity region of the microcrystalline semiconductor filmis irradiated with laser light through the channel protective layer withthe use of the source and drain electrodes as masks.

According to the present invention, a display device including a thinfilm transistor with high electric characteristics and high reliabilitycan be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view describing a display device of the present invention.

FIGS. 2A to 2E are views describing a manufacturing method of a displaydevice of the present invention.

FIGS. 3A to 3C are views describing a manufacturing method of a displaydevice of the present invention.

FIGS. 4A to 4D are views describing a manufacturing method of a displaydevice of the present invention.

FIG. 5 is a view describing a display device of the present invention.

FIGS. 6A to 6D are views describing a manufacturing method of a displaydevice of the present invention.

FIGS. 7A to 7D are diagrams showing electronic devices to which thepresent invention is applied.

FIG. 8 is a block diagram showing a main structure of an electronicdevice to which the present invention is applied.

FIGS. 9A to 9C are views each describing a display device of the presentinvention.

FIGS. 10A and 10B are views describing a display device of the presentinvention.

FIGS. 11A to 11C are views each describing a manufacturing method of adisplay device of the present invention.

FIGS. 12A and 12B are views describing a display device of the presentinvention.

FIGS. 13A and 13B are plan views each describing a plasma CVD apparatusof the present invention.

FIG. 14 is a view describing a display device of the present invention.

FIG. 15 is a view describing a display device of the present invention.

FIGS. 16A and 16B are views of a display device of the presentinvention.

FIG. 17 is a view describing a display device of the present invention.

FIG. 18 is a view describing a display device of the present invention.

FIG. 19 is a view describing a display device of the present invention.

FIG. 20 is a diagram describing a display device of the presentinvention.

FIG. 21 is a view describing a display device of the present invention.

FIG. 22 is a view describing a display device of the present invention.

FIG. 23 is a view describing a display device of the present invention.

FIG. 24 is a diagram describing a display device of the presentinvention.

FIG. 25 is a view describing a display device of the present invention.

FIG. 26 is a view describing a display device of the present invention.

FIG. 27 is a view describing a display device of the present invention.

FIG. 28 is a view describing a display device of the present invention.

FIG. 29 is a view describing a display device of the present invention,

FIG. 30 is a view describing a display device of the present invention.

FIG. 31 is a view describing a manufacturing method of a display deviceof the present invention.

FIGS. 32A to 32C are views describing a manufacturing method of adisplay device of the present invention.

FIGS. 33A to 33C are views describing a manufacturing method of adisplay device of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The embodiment modes according to the present invention will hereinafterbe described in detail, using the accompanying drawings. It is easilyunderstood by those who skilled in the art that the embodiment modes anddetails herein disclosed can be modified in various ways withoutdeparting from the purpose and scope of the present invention.Therefore, the present invention is not construed as being limited tothe description of the following embodiment modes. Note that the sameportions or portions having similar functions are denoted by the samereference numerals throughout the diagrams in structures of the presentinvention described hereinafter, and repetitive description thereof isomitted.

(Embodiment Mode 1)

In this embodiment mode, a thin film transistor which is used for adisplay device and a manufacturing process of the thin film transistorwill be described using FIG. 1, FIGS. 2A to 2E, FIGS. 3A to 3C, andFIGS. 4A to 4D. FIG. 1, FIGS. 2A to 2E, and FIGS. 3A to 3C arecross-sectional views showing a thin film transistor and a manufacturingprocess thereof, and FIGS. 4A to 4D are plan views each showing a regionin a pixel where the thin film transistor and a pixel electrode areconnected to each other. FIG. 1, FIGS. 2A to 2E, and FIGS. 3A to 3C arecross-sectional views showing the thin film transistor along line A-B inFIGS. 4A to 4D, and a manufacturing process thereof.

As for a thin film transistor including a microcrystalline semiconductorfilm, an n-channel thin film transistor has higher mobility than ap-channel thin film transistor; thus, an n-channel thin film transistoris more suitable for a driver circuit. In this embodiment mode, eitheran n-channel thin film transistor or a p-channel thin film transistorcan be used. Regardless of whether the thin film transistor is n-channeltype or p-channel type, it is preferable that all the thin filmtransistors formed over the same substrate have the same conductivitytype so that the number of manufacturing steps is suppressed. In thisembodiment mode, description will be made using an n-channel thin filmtransistor.

A channel-stop-type (also called a channel-protective-type) bottom-gatethin film transistor 74 of this embodiment mode is shown in FIG. 1.

In FIG. 1, the channel-stop-type thin film transistor 74 is providedover a substrate 50. The channel-stop-type thin film transistor 74includes a gate electrode 51, gate insulating films 52 a and 521), amicrocrystalline semiconductor film 61, a buffer layer 62, a channelprotective layer 80, a source and drain regions 72, and source and drainelectrodes 71 a, 71 b, and 71 c. A pixel electrode 77 is provided so asto be in contact with the source and drain electrodes 71 c. Aninsulating film 76 is provided so as to cover the thin film transistor74 and part of the pixel electrode 77. Note that FIG. 1 corresponds toFIG. 4D.

Furthermore, in a channel formation region of the microcrystallinesemiconductor film 61, an impurity region 81 containing an impurityelement of one conductivity type is provided selectively in a regionwhich is not overlapped with the source and drain electrodes 71 a, 71 b,and 71 c.

In this embodiment mode, channel doping is selectively (partly)performed in the channel formation region of the microcrystallinesemiconductor film 61. After the formation of the source and drainelectrodes 71 a, 71 b, and 71 c, an impurity element of one conductivitytype is added to the microcrystalline semiconductor film 61 through thebuffer layer 62 and the channel protective layer 80 which is exposedbetween the source and drain electrodes 71 a, 71 b, and 71 c, with thesource and drain electrodes 71 a, 71 b, and 71 c (or a mask layer) usedas masks (or a mask), whereby an added region and a non-added region ofthe impurity element of one conductivity type are generated in aself-aligned manner in the channel formation region of themicrocrystalline semiconductor film 61 which is covered with the channelprotective layer 80, so that the impurity region 81 can be formedselectively.

The microcrystalline semiconductor film has low n-type conductivity whenan impurity element for controlling valence electrons is not addedthereto intentionally. Therefore, by addition of an impurity elementwhich imparts p-type conductivity to the microcrystalline semiconductorfilm which functions as a channel formation region of a thin filmtransistor, the threshold voltage can be controlled. A typical exampleof the impurity element which imparts p-type conductivity is boron; andan impurity gas such as B₂H₆ or BF₃ may be added to silicon hydride at 1ppm to 1000 ppm, preferably, 1 ppm to 100 ppm. The concentration ofboron may be set to 1×10¹⁴ to 6'10¹⁶ atoms/cm³.

A thin film transistor is a switching element that is turned on when acertain voltage (referred to as a threshold value or a thresholdvoltage) is applied to a gate electrode and is turned off when a voltagewhich is less than the certain voltage is applied. Therefore, it is veryimportant to control a threshold value precisely in terms of accurateoperation of a circuit.

However, the threshold voltage of a TFT may be moved (shifted) in thenegative direction or the positive direction by an indefinite factorsuch as an effect of a movable ion due to contamination or an effect ofdifference in work function or interface electric charge in theperiphery of a gate of the TFT.

As a technique proposed as a means for solving such a phenomenon, thereis a channel doping method. The channel doping method is a technique inwhich an impurity element which imparts one conductivity type(typically, P, As, B, or the like) is added to a channel formationregion of a TFT so that the threshold voltage is shifted intentionallyto be controlled.

In the present invention, channel doping is selectively (partly)performed in a channel formation region of a microcrystallinesemiconductor film. In this specification, an impurity region which isformed in the channel formation region by a channel doping step is alsocalled a channel doping region. After formation of source and drainelectrodes, an impurity element of one conductivity type is added to themicrocrystalline semiconductor film through a buffer layer and a channelprotective layer which is exposed between the source and drainelectrodes, with the source and drain electrodes (or a mask layer) usedas masks (or a mask), whereby an added region and a non-added region ofthe impurity element of one conductivity type are generated in aself-aligned manner in the channel formation region of themicrocrystalline semiconductor film which is covered with the channelprotective layer so that an impurity region can be formed selectively.

The microcrystalline semiconductor film has low n-type conductivity whenan impurity element for controlling valence electrons is not addedthereto intentionally. Therefore, by addition of an impurity elementwhich imparts p-type conductivity to the microcrystalline semiconductorfilm which functions as a channel formation region of a thin filmtransistor, the threshold voltage can be controlled. A typical exampleof the impurity element which imparts p-type conductivity is boron; andan impurity gas such as B₂H₆ or BF₃ may be added to silicon hydride at 1ppm to 1000 ppm, preferably, 1 ppm to 100 ppm. The concentration ofboron may be set to 1×10¹⁴ to 6×10¹⁶ atoms/cm³.

In the present invention, since the addition of the impurity element tothe microcrystalline semiconductor film is performed through the channelprotective layer, damage (e.g., surface roughness) to themicrocrystalline semiconductor film by the addition can be reduced. Notethat, in the present invention, the addition of the impurity element tothe microcrystalline semiconductor film is performed so that a desiredconcentration peak of the impurity element exists in themicrocrystalline semiconductor film, and the impurity element may alsobe added to the buffer layer in the case where the addition of theimpurity element to the microcrystalline semiconductor film is performedthrough the channel protective layer and the buffer layer.

Control of the threshold voltage by a channel doping method is carriedout by the concentration of an impurity element. In this embodimentmode, channel doping is performed not on the whole of the channelformation region but so as to selectively form a channel doping region.Therefore, in the present invention, the threshold voltage can becontrolled more precisely by controlling the area of the channelformation region. In the case where the impurity is added to themicrocrystalline semiconductor film through the channel protectivelayer, it is difficult to control the concentration of the impurityelement in the microcrystalline semiconductor film which exists deep ina film-thickness direction and the control tends to be varied, and thereis a fear of damage to the film because the addition needs to beperformed at energy high enough to pass the impurity element through thechannel protective layer. According to the present invention, filmdamage to the microcrystalline semiconductor film can be prevented andthe threshold value can be controlled more accurately and uniformly.Accordingly, high reliability and high performance can be achieved in athin film transistor and a display device including the thin filmtransistor.

With the structure in which the channel protective layer (also referredto as simply a protective layer) is provided over the channel formationregion of the microcrystalline semiconductor film with the buffer layerinterposed therebetween, which is one mode of the present invention,damage which is caused in the manufacturing process, to the buffer layerover the channel formation region of the microcrystalline semiconductorfilm (e.g., reduction in film thickness due to plasma or an etchingagent in etching, or oxidation) can be prevented. Therefore, reliabilityof a thin film transistor can be improved. Further, since the bufferlayer over the channel formation region of the microcrystallinesemiconductor film is not etched, the buffer layer is not needed to beformed thickly, and thus film deposition time of the buffer layer can beshortened. Note that the channel protective layer functions as anetching stopper in etching for forming a source and drain regions andthus can also be referred to as a channel stopper layer.

As an example of the buffer layer, an amorphous semiconductor film isgiven; it is preferable that an amorphous semiconductor film containingat least one of nitrogen, hydrogen, and halogen be used. By containingat least one of nitrogen, hydrogen, and halogen in the amorphoussemiconductor film, oxidation of crystals included in themicrocrystalline semiconductor film can be reduced. While themicrocrystalline semiconductor film has an energy gap of 1.1 to 1.5 eV,the buffer layer has a large energy gap of 1.6 to 1.8 eV and lowmobility. The mobility of the buffer layer is typically ⅕ to 1/10 assmall as that of the microcrystalline semiconductor film. Thus, thechannel formation region is formed of a microcrystalline semiconductorfilm, and the buffer layer is a high-resistance region. Note that theconcentration of each of carbon, nitrogen, and oxygen contained in themicrocrystalline semiconductor film is set to less than or equal to3×10¹⁹ atoms/cm³, preferably, less than or equal to 5×10¹⁸ atoms/cm³.The thickness of the microcrystalline semiconductor film is preferably 2to 50 nm, more preferably, 10 nm to 30 nm.

The buffer layer can be formed by a plasma CVD method, a sputteringmethod, or the like. Further, a buffer layer can also be formed asfollows; an amorphous semiconductor film is formed, and then, a surfaceof the amorphous semiconductor film is nitrided, hydrogenated, orhalogenated by performing nitrogen plasma treatment, hydrogen plasmatreatment, or halogen plasma treatment to the surface of the amorphoussemiconductor film.

The buffer layer provided on a surface of the microcrystallinesemiconductor film can reduce oxidation of crystal grains included inthe microcrystalline semiconductor film, and thus, deterioration ofelectric characteristics of the thin film transistor can be reduced.

The microcrystalline semiconductor film can be directly formed over asubstrate, unlike a polycrystalline semiconductor film. Specifically,the microcrystalline semiconductor film can be formed using siliconhydride as a source gas with a microwave plasma CVD apparatus with afrequency of 1 GHz or more. The microcrystalline semiconductor filmformed by the above method also includes a microcrystallinesemiconductor film which has crystal grains with a diameter of 0.5 to 20nm in an amorphous semiconductor. Thus, unlike the case of using apolycrystalline semiconductor film, there is no need to perform acrystallization step after formation of a semiconductor film. The numberof steps in manufacturing a thin film transistor can be reduced, a yieldof a display device can be increased, and cost can be suppressed.Further, plasma generated by using microwaves with a frequency of 1 GHzor more has high electron density, and thus, silicon hydride which is asource gas can be easily dissociated. Therefore, compared to a highfrequency plasma CVD method with a frequency of several tens to severalhundreds of megahertz, the microcrystalline semiconductor film can beformed more easily and film deposition rate can be increased. Thus, themass productivity of liquid crystal display devices can be increased.

Further, a thin film transistor (TFT) is manufactured using amicrocrystalline semiconductor film, and a display device ismanufactured using the thin film transistor for a pixel portion, andfurther, for a driver circuit. The thin film transistor using amicrocrystalline semiconductor film has a mobility of 1 to 20 cm²/V·sec,which is 2 to 20 times as high as that of a thin film transistor usingan amorphous semiconductor film. Therefore, part of or the whole of thedriver circuit can be formed over the same substrate as the pixelportion, so that a system-on-panel can be formed.

The gate insulating film, the microcrystalline semiconductor film, thebuffer layer, the channel protective layer, and the semiconductor filmto which an impurity element which imparts one conductivity type isadded to form in a source and drain regions may be formed in either thesame reaction chamber, or different reaction chambers depending on thekind of a film.

Before a substrate is carried into a reaction chamber to perform filmformation, it is preferable to perform cleaning, flush (washing)treatment (e.g., hydrogen flush using hydrogen as a flush substance, orsilane flush using silane as a flush substance), and/or coating by whichthe inner wall of each reaction chamber is coated with a protective film(the coating is also referred to as pre-coating treatment). Pre-coatingtreatment is treatment in which plasma treatment is performed by flowingof a deposition gas in a reaction chamber to coat the inner wall of thereaction chamber with a thin protective film which is a film to beformed, in advance. By the flush treatment and/or the pre-coatingtreatment, a film to be formed can be prevented from being contaminatedby an impurity such as oxygen, nitrogen, or fluorine in the reactionchamber.

Since the channel protective layer 80 is provided over the channelformation region of the microcrystalline semiconductor film 61 with thebuffer layer 62 interposed therebetween, damage (e.g., reduction in filmthickness due to plasma or an etching agent in etching, or oxidation)which is caused in the manufacturing process, to the buffer layer 62over the channel formation region of the microcrystalline semiconductorfilm 61 can be prevented. Therefore, reliability of the thin filmtransistor 74 can be improved. Further, the buffer layer 62 over thechannel formation region of the microcrystalline semiconductor film 61is not etched, so that the buffer layer 62 is not needed to be formedthickly and film deposition time can be shortened.

Hereinafter, a manufacturing method will be described in detail. Thegate electrode 51 is formed over the substrate 50 (see FIGS. 2A and 4A).FIG. 2A is a cross-sectional view taken along line A-B in FIG. 4A. Asthe substrate 50, a plastic substrate having sufficient heat resistanceto withstand a processing temperature of a manufacturing process, or thelike as well as a non-alkaline glass substrate manufactured by a fusionmethod or a float method, such as a substrate of barium borosilicateglass, aluminoborosilicate glass, or aluminosilicate glass, or a ceramicsubstrate can be used. Alternatively, a metal substrate such as astainless steel alloy substrate which is provided with an insulatingfilm on its surface may be used. As for the size of the substrate 50,the following can be given; 320 mm×400 mm, 370 mm×470 mm, 550 mm×650 mm,600 mm×720 mm, 680 mm×880 mm, 730 mm×920 mm, 1000 mm×1200 mm, 1100mm×1250 mm, 1150 mm×1300 mm, 1500 mm×1800 mm, 1900 mm×2200 mm, 2160mm×2460 mm, 2400 mm×2800 mm, 2850 mm×3050 mm, or the like.

The gate electrode 51 is formed of a metal material such as titanium,molybdenum, chromium, tantalum, tungsten, or aluminum, or an alloymaterial thereof. The gate electrode 51 can be formed in the followingmanner: a conductive film is formed over the substrate 50 by asputtering method or a vacuum evaporation method, a mask is formed by aphotolithography technique or an ink-jet method over the conductivefilm, and the conductive film is etched using the mask. Alternatively,the gate electrode 51 can be formed by discharging a conductivenanopaste of silver, gold, copper, or the like by an ink-jet method andbaking it. Note that, as barrier metal which improve adhesion of thegate electrode 51 and prevents diffusion to a base and a substrate, anitride film of the above-described metal material may be providedbetween the substrate 50 and the gate electrode 51. Further, the gateelectrode 51 may have a stacked-layer structure; a structure can be usedin which, from the substrate 50 side, an aluminum film and a molybdenumfilm are stacked, a copper film and a molybdenum film are stacked, acopper film and a titanium nitride film are stacked, a copper film and atantalum nitride film are stacked, or the like. In the above-describedstacked-layer structure, the molybdenum film or the nitride film such asthe titanium nitride film or the tantalum nitride film which is formedas the upper layer has an effect as a barrier metal.

Note that, since a semiconductor film and/or a wiring are/is formed overthe gate electrode 51, the gate electrode 51 is preferably processed tohave a tapered end portion so that the semiconductor film and/or thewiring thereover are/is not disconnected due to a step. Further,although not shown, a wiring connected to the gate electrode can also beformed at the same time when the gate electrode 51 is formed.

Next, the gate insulating films 52 a and 52 b, a microcrystallinesemiconductor film 53, and a buffer layer 54 are formed in sequence overthe gate electrode 51 (see FIG. 2B).

The microcrystalline semiconductor film 53 may be formed on the surfaceof the gate insulating film 52 b either while being subjected tohydrogen plasma or after the gate insulating film 52 h is subjected tohydrogen plasma. By forming the microcrystalline semiconductor film overthe gate insulating film which has been subjected to hydrogen plasma,crystal growth of microcrystals can be promoted. Further, latticedistortion at an interface between the gate insulating film and themicrocrystalline semiconductor film can be reduced, and thus, interfacecharacteristics of the gate insulating film and the microcrystallinesemiconductor film can be improved. Consequently, a microcrystallinesemiconductor film with excellent electric characteristics and highreliability can he obtained.

Note that the gate insulating films 52 a and 52 b, the microcrystallinesemiconductor film 53, and the buffer layer 54 may be formedsuccessively without being exposed to the air. By successively formingthe gate insulating films 52 a and 52 b, the microcrystallinesemiconductor film 53, and the buffer layer 54 without being exposed tothe air, the films can be formed without any contamination of eachinterface between the films with an air constituent or an impurityelement floating in the air. Thus, variations in characteristics of thinfilm transistors can be reduced.

The gate insulating films 52 a and 52 b can each be formed by a CVDmethod, a sputtering method, or the like using a silicon oxide film, asilicon nitride film, a silicon oxynitride film, or a silicon nitrideoxide film. In this embodiment mode, a mode in which either a siliconnitride film or a silicon nitride oxide film and either a silicon oxidefilm or a silicon oxynitride film are stacked sequentially as the gateinsulating film 52 a and the gate insulating film 52 b, respectively isdescribed. Note that the gate insulating film is not limited to have atwo-layer structure but may have a three-layer structure in which eithera silicon nitride film or a silicon nitride oxide film, either a siliconoxide film or a silicon oxynitride film, and either a silicon nitridefilm or a silicon nitride oxide film are stacked sequentially from thesubstrate side. Alternatively, the gate insulating film can be formed ofa single layer of a silicon oxide film, a silicon nitride film, asilicon oxynitride film, or a silicon nitride oxide film. Furthermore,the gate insulating film is preferably formed by a microwave plasma CVDapparatus with a frequency of 1 GHz or more. A silicon oxynitride filmand a silicon nitride oxide film which are formed by a microwave plasmaCVD apparatus each have high withstand voltage, and thus, reliability ofa thin film transistor can be improved.

As an example of the three-layer structure of the gate insulating film,over the gate electrode, a silicon nitride film or a silicon nitrideoxide film may be formed as a first layer, a silicon oxynitride film maybe formed as a second layer, and a silicon nitride film may be formed asa third layer. The microcrystalline semiconductor film may be formedover the silicon nitride film that is a top layer. In this case, thesilicon nitride film or the silicon nitride oxide film that is the firstlayer is preferably thicker than 50 nm and has an effect as a barrierwhich blocks impurities such as sodium, an effect of preventing ahillock of the gate electrode, an effect of preventing oxidation of thegate electrode, and the like. The silicon nitride film that is the thirdlayer has an effect of improving adherence of the microcrystallinesemiconductor film and an effect of preventing oxidation in LP treatmentin which the microcrystalline semiconductor film is irradiated with alaser beam.

By forming a nitride film such as a silicon nitride film which is verythin as the top layer of the gate insulating film, as described above,adherence of the microcrystalline semiconductor film can be improved.The nitride film may be formed by a plasma CVD method, or by nitridationtreatment performed by treatment with plasma which is generated bymicrowaves and has high density and low temperature. Further, a siliconnitride film or a silicon nitride oxide film may be formed when areaction chamber is subjected to silane flush treatment.

In this specification, the silicon oxynitride film means a film thatcontains more oxygen than nitrogen and includes oxygen, nitrogen,silicon, and hydrogen at concentrations ranging from 55 at. % to 65 at.%, 1 at. % to 20 at. %, 25 at. % to 35 at. %, and 0.1 at. % to 10 at. %,respectively. Further, the silicon nitride oxide film means a film thatcontains more nitrogen than oxygen and includes oxygen, nitrogen,silicon, and hydrogen at concentrations ranging from 15 at. % to 30 at.%, 20 at. % to 35 at. %, 25 at. % to 35 at. %, and 15 at. % to 25 at. %,respectively.

The microcrystalline semiconductor film 53 is a film which contains asemiconductor having an intermediate structure between amorphous andcrystalline structures (including a single crystal and a polycrystal).This semiconductor is a semiconductor which has a third state that isstable in terms of free energy, and is a crystalline semiconductor whichhas short-range order and lattice distortion, and column-like orneedle-like crystals with a grain size, seen from the film surface, of0.5 to 20 nm grown in the direction of a normal line with respect to thesurface of the substrate. In addition, a microcrystalline semiconductorand an amorphous semiconductor are mixed. Microcrystalline silicon,which is a typical example of a microcrystalline semiconductor, has aRaman spectrum which is shifted to a lower wave number side than 521cm⁻¹ that represents single-crystalline silicon. That is, the peak of aRaman spectrum of microcrystalline silicon exists between 521 cm⁻¹ thatrepresents single-crystalline silicon and 480 cm⁻¹ that representsamorphous silicon. Further, microcrystalline silicon is made to containhydrogen or halogen at at least 1 at. % for termination of danglingbonds. Moreover, a rare gas element such as helium, argon, krypton, orneon may be included to further promote lattice distortion, so that thestability is enhanced and a favorable microcrystalline semiconductorfilm can be obtained. Such a microcrystalline semiconductor film isdisclosed in, for example, U.S. Pat. No. 4,409,134.

The microcrystalline semiconductor film can be formed by ahigh-frequency plasma CVD method with a frequency of several tens toseveral hundreds of megahertz or a microwave plasma CVD method with afrequency of 1 GHz or more. The microcrystalline semiconductor film canbe typically formed by a dilution of silicon hydride such as SiH₄,Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, or SiF₄ with hydrogen. Further, by adilution with one or plural kinds of rare gas elements selected fromhelium, argon, krypton, and neon in addition to silicon hydride andhydrogen, the microcrystalline semiconductor film can be fainted aswell. The flow ratio of hydrogen to silicon hydride at this time is setto 5:1 to 200:1, preferably, 50:1 to 150:1, more preferably, 100:1.

Preferably, the microcrystalline semiconductor film contains oxygen at aconcentration of 5×10¹⁹ atoms/cm³ or less, more preferably, 1×10¹⁹atoms/cm³ or less, and nitrogen and carbon each at a concentration of1×10¹⁸ atoms/cm³ or less. By decreasing the concentrations of oxygen,nitrogen, and carbon in the microcrystalline semiconductor film, themicrocrystalline semiconductor film can be prevented from having n-typeconductivity.

The microcrystalline semiconductor film 53 is formed with a thicknesswhich is greater than 0 nm and less than or equal to 50 nm, preferably,greater than 0 nm and less than or equal to 20 nm.

The microcrystalline semiconductor film 53 functions as a channelformation region of a thin film transistor. By forming themicrocrystalline semiconductor film 53 to have a thickness within theabove-described range, a fully depleted thin film transistor can beformed. Further, since the microcrystalline semiconductor film is formedof microcrystals, the microcrystalline semiconductor film has lowerresistance than an amorphous semiconductor film. Therefore, a thin filmtransistor using the microcrystalline semiconductor film hascurrent-voltage characteristics represented by a curve with a steepslope in a rising portion, has an excellent response as a switchingelement, and can be operated at high speed. Further, with the use of themicrocrystalline semiconductor film in a channel formation region of athin film transistor, fluctuation of a threshold value of a thin filmtransistor can be suppressed. Therefore, a display device with lessvariation of electrical characteristics can be manufactured.

Further, the microcrystalline semiconductor film has a higher mobilitythan an amorphous semiconductor film. Thus, by using, as a switch of adisplay element, a thin film transistor in which a channel formationregion is formed of a microcrystalline semiconductor film, the area ofthe channel formation region, that is, the area of the thin filmtransistor can be reduced. Accordingly, the area occupied by the thinfilm transistor per pixel is decreased, and an aperture ratio of thepixel can be increased. As a result of this, a device with highresolution can be manufactured.

Further, the microcrystalline semiconductor film has a needle-likecrystal which has grown longitudinally from the lower side. Themicrocrystalline semiconductor film has amorphous and crystallinestructures which are mixed, and it is likely that a crack is generatedand a gap is formed between the crystalline region and the amorphousregion due to local stress. A radical may be interposed into this gapand cause crystal growth. Because the upper crystal face becomes larger,crystals are likely to grow upward into a needle shape. Even if themicrocrystalline semiconductor film grows longitudinally as describedabove, the deposition rate is one-tenth to one-hundredth as large asthat of an amorphous semiconductor film.

The buffer layer 54 can be formed by a plasma CVD method using a silicongas (a silicon hydride gas or a silicon halide gas) such as SiH₄, Si₂H₆,SiH₂Cl₂, SiHCl₃, SiCl₄, or SiF₄. Alternatively, a dilution of silanementioned above with one or plural kinds of rare gas elements selectedfrom helium, argon, krypton, and neon can be used to form an amorphoussemiconductor film. Further alternatively, an amorphous semiconductorfilm containing hydrogen can be formed using hydrogen with a flow rateof 1 to 20 times inclusive, preferably, 1 to 10 times inclusive, morepreferably, 1 to 5 times inclusive as high as that of silicon hydride.Further alternatively, an amorphous semiconductor film containingnitrogen can be formed using the silicon hydride and either nitrogen orammonia. Further alternatively, an amorphous semiconductor filmcontaining fluorine, chlorine, bromine, or iodine can be formed usingthe above silicon hydride, and a gas containing fluorine, chlorine,bromine, or iodine (e.g., F₂, Cl₂, Br₂, I₂, HF, HCl, HBr, or HI).

Alternatively, as the buffer layer 54, an amorphous semiconductor filmcan be formed by sputtering with hydrogen or a rare gas, using anamorphous semiconductor as a target. At this time, by containingammonia, nitrogen, or N₂O in the atmosphere, an amorphous semiconductorfilm containing nitrogen can be formed. Further alternatively, bycontaining a gas containing fluorine, chlorine, bromine, or iodine(e.g., F₂, Cl₂, Br₂, I₂, HF, HCl, HBr, or HI) in the atmosphere, anamorphous semiconductor film containing fluorine, chlorine, bromine, oriodine can be formed.

Further alternatively, the buffer layer 54 may be formed as follows: anamorphous semiconductor film is formed on the surface of themicrocrystalline semiconductor film 53 by a plasma CVD method or asputtering method and then the surface of the amorphous semiconductorfilm is hydrogenised, nitrided, or halogenated by performing plasmatreatment with hydrogen plasma, nitrogen plasma, halogen plasma, orplasma of a rare gas (helium, argon, krypton, or neon) on the surface ofthe amorphous semiconductor film.

The buffer layer 54 is preferably formed of an amorphous semiconductorfilm. Therefore, if the buffer layer 54 is formed by a high-frequencyplasma CVD method with a frequency of several tens to several hundredsof megahertzs, or a microwave plasma CVD method, it is preferable tocontrol the film deposition condition so that an amorphous semiconductorfilm can be formed as the buffer layer 54.

The buffer layer 54 is preferably formed with a thickness greater thanor equal to 10 nm and equal to or less than 50 nm. Further, since thebuffer layer 54 over the channel formation region of themicrocrystalline semiconductor film 53 is not etched, the buffer layer54 is not needed to be formed thickly, and thus film deposition time ofthe buffer layer can be shortened. Further, the total concentration ofnitrogen, carbon, and oxygen contained in the buffer layer is preferablyset to 1×10²⁰ atoms/cm³ to 1.5×10²¹ atoms/cm³. With this concentration,the buffer layer 54 can function as a high-resistance region even whenthe thickness is greater than or equal to 10 nm and equal to or lessthan 50 nm.

The buffer layer 54 may be formed with a thickness greater than or equalto 150 nm and equal to or less than 200 nm, and the concentration ofeach of carbon, nitrogen, and oxygen contained in the buffer layer 54may be set to less than or equal to 3×10¹⁹ atoms/cm³, preferably, lessthan or equal to 5×10¹⁸ atoms/cm³.

By forming, as a buffer layer, an amorphous semiconductor film or anamorphous semiconductor film containing hydrogen, nitrogen, or halogenon the surface of the microcrystalline semiconductor film 53, thesurfaces of crystal grains contained in the microcrystallinesemiconductor film 53 can be prevented from being naturally oxidized.That is, by formation of the buffer layer on the surface of themicrocrystalline semiconductor film 53, microcrystal grains can beprevented from being oxidized. Since the buffer layer includes hydrogenand/or fluorine, oxygen can be prevented from entering themicrocrystalline semiconductor film.

Further, the buffer layer 54, which is formed of an amorphoussemiconductor film or an amorphous semiconductor film containinghydrogen, nitrogen, or halogen, has higher resistance than themicrocrystalline semiconductor film which functions as a channelformation region. Therefore, in a thin film transistor, the buffer layerwhich is formed between a source and drain regions and themicrocrystalline semiconductor film functions as a high-resistanceregion. Accordingly, the off current of the thin film transistor can bereduced. When the thin film transistor is used as a switching element ofa display device, the contrast of the display device can be improved.

Next, the channel protective layer 80 is formed over the buffer layer 54so as to overlap with the channel formation region of themicrocrystalline semiconductor film 53 (see FIG. 2C). The channelprotective layer 80 may also be formed successively after the gateinsulating films 52 a and 52 b, the microcrystalline semiconductor film53, and the buffer layer 54 are formed, without being exposed to theair. Successive formation of the thin films stacked improvesproductivity.

The channel protective layer 80 can be formed of an inorganic material(e.g., silicon oxide, silicon nitride, silicon oxynitride, or siliconnitride oxide). Alternatively, a photosensitive or non-photosensitiveorganic material (organic resin material, such as polyimide, acrylic,polyamide, polyimideamide, resist, or benzocyclobutene), a film made ofplural kinds of these materials, or a stacked-layer film of them may beused. Further alternatively, siloxane may be used. As a formationmethod, a vapor deposition method such as a plasma CVD method or athermal CVD method, or a sputtering method can be used. Alternatively, acoating method such as a spin coating method or a droplet dischargingmethod which is a wet method, a printing method (such as screen printingor offset printing by which a pattern is formed), or the like can beused. The channel protective layer 80 may be patterned by etching afterthe film formation, or may be formed selectively by a dropletdischarging method or the like.

Next, the microcrystalline semiconductor film 53 and the buffer layer 54are patterned by etching, thereby forming a stacked-layer of themicrocrystalline semiconductor film 61 and the buffer layer 62 (see FIG.2D). The microcrystalline semiconductor film 61 and the buffer layer 62can he formed by fowling a mask by a photolithography technique or adroplet discharging method and etching the microcrystallinesemiconductor film 53 and the buffer layer 54 with the use of the mask.Note that FIG. 20 is a cross-sectional view taken along line A-B in FIG.4B.

The end portions of the microcrystalline semiconductor film 61 and thebuffer layer 62 are etched to have tapered shapes. The angle of each ofthe end portions is 30° to 90°, preferably, 45° to 80°. Thus,disconnection of a wiring due to a step can be prevented.

Next, the semiconductor film 63 to which an impurity element whichimparts one conductivity type is added and conductive films 65 a to 65 care formed over the gate insulating film 52 b, the microcrystallinesemiconductor film 61, the buffer layer 62, and the channel protectivelayer 80 (see FIG. 2E). A mask 66 is formed over the semiconductor film63 to which an impurity which imparts one conductivity type is added andthe conductive films 65 a to 65 c. The mask 66 is formed by aphotolithography technique or an ink-jet method.

In the case where an n-channel thin film transistor is formed,phosphorus may be added as a typical impurity element to thesemiconductor film 63 to which an impurity which imparts oneconductivity type is added; for example, an impurity gas such as PH₃ isadded to silicon hydride. On the other hand, in the case where ap-channel thin film transistor is formed, boron may be added as atypical impurity element to the semiconductor film 63 to which animpurity which imparts one conductivity type is added; for example, animpurity gas such as B₂H₆ is added to silicon hydride. The semiconductorfilm 63 to which an impurity which imparts one conductivity type isadded can he formed of a microcrystalline semiconductor or an amorphoussemiconductor. The semiconductor film 63 to which an impurity whichimparts one conductivity type is added is preferably formed to have athickness of 2 to 50 nm (preferably, 10 to 30 nm).

The conductive films are each preferably formed of a single layer or astacked layer of aluminum, copper, and/or an aluminum alloy to which anclement to improve heat resistance or an element to prevent a hillocksuch as silicon, titanium, neodymium, scandium, or molybdenum is added.Further, a stacked-layer structure in which a film in contact with thesemiconductor film to which an impurity which imparts one conductivitytype is added is formed of titanium, tantalum, molybdenum, or tungsten,or nitride of such an element, and aluminum or an aluminum alloy isformed thereover may also be used. Further alternatively, astacked-layer structure in which an aluminum film or an aluminum alloyfilm is sandwiched between upper and lower films of titanium, tantalum,molybdenum, tungsten, or nitride of any of these elements may be used.In this embodiment mode, a conductive film having a structure in whichthree layers of the conductive films 65 a and 65 c are stacked isdescribed as the conductive film; for example, a stacked-layerconductive film in which molybdenum films are used as the conductivefilms 65 a and 65 c and an aluminum film is used as the conductive film65 b or a stacked-layer conductive film in which titanium films are usedas the conductive films 65 a and 65 c and an aluminum film is used asthe conductive film 65 b can be given.

The conductive films 65 a to 65 c are formed by a sputtering method or avacuum evaporation method. Alternatively, the conductive films 65 a to65 c may be formed by discharging a conductive nanopaste of silver,gold, copper, or the like by a screen printing method, an inkjet method,or the like and baking it.

Next, the conductive films 65 a to 65 c are etched using the mask 66 toform the source and drain electrodes 71 a to 71 c. When the conductivefilms 65 a to 65 c are subjected to wet etching as shown in FIGS. 3A to3C in this embodiment mode, the conductive films 65 a to 65 c areisotropically etched, so that end portions of the source and drainelectrodes 71 a to 71 c are not aligned with and are laid back from endportions of the mask 66. Next, the semiconductor film 63 to which animpurity which imparts one conductivity type is added is etched usingthe mask 66 to form the source and drain regions 72 (see FIG. 3A). Notethat the buffer layer 62 is not etched because the channel protectivelayer 80 functions as a channel stopper.

The end portions of the source and drain electrodes 71 a to 71 c arc notaligned with the end portions of the source and drain regions 72, andthe end portions of the source and drain regions 72 are formed outsideof the end portions of the source and drain electrodes 71 a to 71 c.

With the use of the mask 66 as a mask, an impurity element 82 is addedto the microcrystalline semiconductor film 61 through the channelprotective layer 80 and the buffer layer 62. By the addition of theimpurity element 82, the impurity region 81 is selectively formed in thechannel formation region of the microcrystalline semiconductor film 61.Since the channel formation region of the microcrystalline semiconductorfilm 61 is covered with the channel protective layer 80, the impurityregion 81 which is a channel doping region is selectively formed in thechannel formation region. The impurity element 82 can be added(introduced) by an ion implantation method or an ion doping method.

The addition of the impurity element 82 may also be performed using thesource and drain electrodes 71 a to 71 c as masks after the mask 66 isremoved, so that a channel doping region can be selectively formed inthe channel formation region of the microcrystalline semiconductor film61 in a self-aligned manner. Alternatively, the impurity region 81 maybe formed using a mask such as a resist mask before the source and drainelectrodes 71 a to 71 c are formed.

In this embodiment mode, since the addition of the impurity element tothe microcrystalline semiconductor film 61 is performed through thechannel protective layer 80 and the buffer layer 62, damage (e.g.,surface roughness) to the microcrystalline semiconductor film by theaddition can be reduced. Note that, in the present invention, theaddition of the impurity element to the microcrystalline semiconductorfilm 61 is performed so that a desired concentration peak of theimpurity element exists in the microcrystalline semiconductor film 61,and the impurity element may also be added to the buffer layer 62 in thecase where the addition of the impurity element to the microcrystallinesemiconductor film 61 is performed through the channel protective layer80 and the buffer layer 62.

After that, the mask 66 is removed. Note that FIG. 3C is across-sectional view taken along line A-B in FIG. 4C. It can be seenfrom FIG. 4C that the end portions of the source and drain regions 72are positioned outside of the end portions of the source and drainelectrodes 71 c. Further, it can also be seen that the area of thesource and drain regions 72 is larger than that of each of the sourceand drain electrodes 71 a to 71 c. Further, one of the source and drainelectrodes also functions as a source or drain wiring.

With a shape as shown in FIG. 3C in which the end portions of the sourceand drain electrodes 71 a to 71 c are not aligned with the end portionsof the source and drain regions 72, the end portions of the source anddrain electrodes 71 a to 71 c are separated from each other; therefore,leakage current and short-circuiting between the source and drainelectrodes can be prevented. Further, the source and drain regions 72extend over the end portions of the source and drain electrodes 71 a to71 c, so that the distance between the pair of the source and drainregions 72 is less than that between each pair of the source and drainelectrodes 71 a to 71 c. Accordingly, a thin film transistor with highreliability and high withstand voltage can be manufactured.

Through the above-described process, the channel-stop-type(protective-type) thin film transistor 74 can be formed.

A portion of the buffer layer 62, which is in contact with the sourceand drain regions 72, and a portion of the buffer layer 62, which is incontact with the channel formation region of the microcrystallinesemiconductor film 61 are a continuous film formed of the same materialat the same time. The buffer layer 62 over the microcrystallinesemiconductor film 61 blocks external air and an etching residue withhydrogen contained therein and protects the microcrystallinesemiconductor film 61.

The buffer layer 62 which does not contain an impurity which imparts oneconductivity type is provided, whereby an impurity which imparts oneconductivity type, contained in the source and drain regions, and animpurity which imparts one conductivity type, used for controllingthreshold voltage of the microcrystalline semiconductor film 61, can beprevented from being mixed with each other. If the impurities each ofwhich impart one conductivity type are mixed with each other, arecombination center is formed, which leads to flow of leakage currentand loss of the effect of reducing off current.

By provision of the buffer layer and the channel protective layer asdescribed above, a channel-stop-type thin film transistor with highwithstand voltage, in which leakage current is reduced, can bemanufactured. Accordingly, the thin film transistor has high reliabilityand can be suitably used for a liquid crystal display device where avoltage of 15 V is applied.

Next, the pixel electrode 77 is formed so as to be in contact with thesource or drain electrode 71 c. The insulating film 76 is formed overthe source and drain electrodes 71 a to 71 c, the source and drainregions 72, the channel protective layer 80, the gate insulating film 52b, and the pixel electrode 77. The insulating film 76 can be formed in asimilar manner to the gate insulating films 52 a and 52 b. Note that theinsulating film 76 is provided to prevent entry of a contaminantimpurity such as an organic substance, a metal substance, or moisturefloating in the air and is preferably a dense film.

The buffer layer 62 is preferably formed with a thickness greater thanor equal to 10 nm and equal to or less than 50 nm. Further, the totalconcentration of nitrogen, carbon, and oxygen contained in the bufferlayer is preferably set to 1×10²° atoms/cm³ to 1.5×10²¹ atoms/cm³. Withthis concentration, the buffer layer 62 can function as ahigh-resistance region even when the thickness is greater than or equalto 10 nm and equal to or less than 50 nm.

Alternately, the buffer layer 62 may be formed with a thickness greaterthan or equal to 150 nm and equal to or less than 200 nm, and theconcentration of each of carbon, nitrogen, and oxygen contained in thebuffer layer 62 may be set to less than or equal to 3×10¹⁹ atoms/cm³,preferably, less than or equal to 5×10¹⁸ atoms/cm³. In this case, byusing a silicon nitride film as the insulating film 76, the oxygenconcentration in the buffer layer 62 can be less than or equal to 5×10¹⁹atoms/cm³, preferably, less than or equal to 1×10¹⁹ atoms/cm³.

Next, the insulating film 76 is etched so that part of the pixelelectrode 77 is exposed. A display element is formed to be in contactwith an exposed region of the pixel electrode 77, so that the thin filmtransistor 74 and the display element can be electrically connected toeach other. For example, a light-emitting layer may be formed over thepixel electrode 77, and a counter electrode may be formed over thelight-emitting layer.

The pixel electrode 77 can be formed of a light-transmitting conductivematerial such as indium oxide containing tungsten oxide, indium zincoxide containing tungsten oxide, indium oxide containing titanium oxide,indium tin oxide containing titanium oxide, indium tin oxide(hereinafter referred to as ITO), indium zinc oxide, or indium tin oxideto which silicon oxide is added.

Alternatively, the pixel electrode 77 can be formed of a conductivecomposition containing a conductive high molecule (also referred to as aconductive polymer). It is preferable that the pixel electrode formed ofa conductive composition have a sheet resistance less than or equal to10000 Ω/square and a light transmittance equal to or greater than 70% ata wavelength of 550 nm. In addition, the resistance of the conductivehigh molecule which is contained in the conductive composition isdesirably less than or equal to 0.1 Ω·cm.

As the conductive high molecule, a so-called it electron conjugated highmolecule can be used. For example, polyaniline or a derivative thereof,polypyrrole or a derivative thereof, polythiophenc or a derivativethereof, and a copolymer of two or more kinds of them can be given.

Further, a shape in which the end portions of a source and drain regionsand the end portions of source and drain electrodes are aligned witheach other may also be employed. A channel-stop-type thin filmtransistor 79 in which the end portions of a source and drain regionsand the end portions of source and drain electrodes are aligned witheach other is shown in FIG. 14. By performing etching of the source anddrain electrodes and the source and drain regions with dry etching, thethin film transistor 79 with the above-described shape can be obtained.Further, by etching a semiconductor film to which an impurity whichimparts one conductivity type is added, with the source and drainelectrodes as masks to form the source and drain regions, the thin filmtransistor 79 with the above-described shape can also be obtained.

A display device includes a display element. A liquid crystal element(also referred to as a liquid crystal display element) or alight-emitting element (also referred to as a light-emitting displayelement) can be used as the display element. The light-emitting elementincludes, in its category, an element whose luminance is controlled by acurrent or voltage, and specifically includes an inorganicelectroluminescent (EL) element, an organic EL element, and the like.Further, a display medium whose contrast is changed by an electriceffect, such as an electronic ink, can be applied as well.

Further, the display device includes a panel in which a display elementis sealed, and a module in which an IC and the like including acontroller are mounted on the panel. The present invention furtherrelates to one mode before the display element is completed in amanufacturing process of the display device, i.e., an element substrate,and the element substrate is provided with a means for supplying currentto the display element for each pixel. The element substrate may bespecifically in a state where only a pixel electrode of a displayelement is formed or in a state after a conductive film to be a pixelelectrode is formed and before the conductive film is etched to form apixel electrode, and can have any mode.

Note that a display device in this specification means an image displaydevice, a display device, or a light source (including a lightingdevice). Further, the display device includes any of the followingmodules in its category: a module including a connector such as anflexible printed circuit (FPC), a tape automated bonding (TAB) tape, ora tape carrier package (TCP); a module having a TAB tape or a TCP whichis provided with a printed wiring board at the end thereof; and a modulehaving an integrated circuit (IC) which is directly mounted on a displayelement by a chip on glass (COG) method.

By forming a channel-stop-type thin film transistor as a thin filmtransistor, reliability of the thin film transistor can be improved.Further, by forming a channel formation region with the use of amicrocrystalline semiconductor film, a field-effect mobility of 1 to 20cm²/V·sec can be achieved. Accordingly, this thin film transistor can beused as a switching element of a pixel in a pixel portion and as anelement included in a scanning line (gate line) driver circuit.

According to this embodiment mode, a display device having a thin filmtransistor with high electric characteristics and high reliability canbe manufactured.

(Embodiment Mode 2)

An example in which the shape of a thin film transistor is different inEmbodiment Mode 1 will be described in this embodiment mode. Except theshape, the thin film transistor can be formed in a similar manner toEmbodiment Mode 1; thus, repetitive description of the same componentsas or components having similar functions to Embodiment Mode 1 andmanufacturing steps for forming those components will be omitted.

A thin film transistor which is used for a display device and amanufacturing process of the thin film transistor will be described withreference to FIG. 5, FIGS. 6A to 6D, and FIG. 15 in this embodimentmode. FIG. 5 and FIG. 15 are cross-sectional views showing a thin filmtransistor and a pixel electrode, and FIGS. 6A to 6D are plane viewsshowing a region in a pixel where the thin film transistor and the pixelelectrode are connected to each other. FIG. 5 and FIG. 15 arecross-sectional views showing the thin film transistor along line Q-R inFIGS. 6A to 6D, and a manufacturing process thereof.

A channel-stop-type (also called a channel-protective-type) bottom-gatethin film transistor 274 of this embodiment mode is shown in FIG. 5 andFIGS. 6A to 6D.

In FIG. 5, the channel-stop-type thin film transistor 274 is providedover a substrate 250. The channel-stop-type thin film transistor 274includes a gate electrode 251, gate insulating films 252 a and 252 b, amicrocrystalline semiconductor film 261, a buffer layer 262, a channelprotective layer 280, a source and drain regions 272, and source anddrain electrodes 271 a, 271 b, and 271 c. An insulating film 276 isprovided so as to cover the thin film transistor 274. A pixel electrode277 is provided so as to be in contact with the source or drainelectrode 271 c in a contact hole formed in the insulating film 276.Note that FIG. 5 corresponds to FIG. 6D.

Furthermore, in a channel formation region of the microcrystallinesemiconductor film 261, an impurity region 281 containing an impurityelement of one conductivity type is provided selectively in a regionwhich is not overlapped with the source and drain electrodes 271 a, 271b, and 271 c.

In this embodiment mode, channel doping is selectively (partly)performed in the channel formation region of the microcrystallinesemiconductor film 261. After the formation of the source and drainelectrodes 271 a, 271 b, and 271 c, an impurity element of oneconductivity type is added to the microcrystalline semiconductor film261 through the buffer layer 262 and the channel protective layer 280which is exposed between the source and drain electrodes 271 a, 271 b,and 271 c, with the source and drain electrodes 271 a, 271 b, and 271 c(or a mask layer) used as masks (or a mask), whereby an added region anda non-added region of the impurity element of one conductivity type aregenerated in a self-aligned manner in the channel formation region ofthe microcrystalline semiconductor film 261 which is covered with thechannel protective layer 280, so that the impurity region 281 can formedselectively.

The microcrystalline semiconductor film has low n-type conductivity whenan impurity element for controlling valence electrons is not addedthereto intentionally. Therefore, by addition of an impurity elementwhich imparts p-type conductivity to the microcrystalline semiconductorfilm which functions as a channel formation region of a thin filmtransistor, the threshold voltage can he controlled. A typical exampleof the impurity element which imparts p-type conductivity is boron; andan impurity gas such as B₂H₆ or BF₃ may he added to silicon hydride at 1ppm to 1000 ppm, preferably, 1 ppm to 100 ppm. The concentration ofboron may be set to 1×10¹⁴ to 6×10¹⁶ atoms/cm³.

Control of the threshold voltage by a channel doping method is carriedout by the concentration of an impurity element. In the presentinvention, channel doping is performed not on the whole of the channelformation region but so as to selectively form a channel doping region.Therefore, in the present invention, the threshold voltage can becontrolled more precisely by controlling the area of the channelformation region. In the case where the impurity is added to themicrocrystalline semiconductor film through the channel protectivelayer, it is difficult to control the concentration of the impurityelement in the microcrystalline semiconductor film which exists deep ina film-thickness direction and the control tends to be varied, and thereis a fear of damage to the film because the addition needs to beperformed at energy high enough to pass the impurity element through thechannel protective layer. According to the present invention, filmdamage to the microcrystalline semiconductor film can be prevented andthe threshold value can be controlled more accurately and uniformly.Accordingly, high reliability and high performance can be achieved in athin film transistor and a display device including the thin filmtransistor.

With the structure in which the channel protective layer 280 is providedover the channel formation region of the microcrystalline semiconductorfilm 261 with the buffer layer 262 interposed therebetween, damage whichis caused in the manufacturing process, to the buffer layer 262 over thechannel formation region of the microcrystalline semiconductor film 261(e.g., reduction in film thickness due to a plasma radical or an etchingagent in etching, or oxidation) can be prevented. Therefore, reliabilityof the thin film transistor 274 can be improved. Further, since thebuffer layer 262 over the channel formation region of themicrocrystalline semiconductor film 261 is not etched, the buffer layer262 is not needed to be formed thickly, and thus film deposition time ofthe buffer layer can be shortened.

Hereinafter, a manufacturing method thereof will be described withreference to FIGS. 6A to 6D. The gate electrode 251 is formed over thesubstrate 250 (see FIG. 6A). The gate insulating films 252 a and 252 bare formed over the gate electrode 251, and the microcrystallinesemiconductor film 261 and the buffer layer 262 are formed thereover.Over the buffer layer 262, the channel protective layer 280 is formed soas to overlap with the channel formation region of the microcrystallinesemiconductor film 261 (see FIG. 6B).

An example in which, after formation of the channel protective layer 80,the microcrystalline semiconductor film 53 and the buffer layer 54 areprocessed into the island-shaped microcrystalline semiconductor film 61and the island-shaped buffer layer 62, respectively, by etching isdescribed in Embodiment Mode 1. On the other hand, an example in whichthe microcrystalline semiconductor film and the buffer layer are etchedat the same time when source and drain electrodes and a semiconductorfilm to which an impurity which imparts one conductivity type is addedare etched will be described in this embodiment mode. Therefore, themicrocrystalline semiconductor film, the buffer layer, the semiconductorfilm to which an impurity which imparts one conductivity type is added,and the source and drain electrodes are formed while reflecting the sameshape. If they are etched by one etching process as described above, themanufacturing process can be simplified, and the number of masks used inthe etching process can be reduced.

A microcrystalline semiconductor film, a buffer layer, a semiconductorfilm to which an impurity which imparts one conductivity type is added,and conductive films are etched, so that the microcrystallinesemiconductor film 261, the buffer layer 262, the source and drainregions 272, and the source and drain electrodes 271 a to 271 c areformed.

An impurity element which imparts one conductivity type is added to themicrocrystalline semiconductor film 261 with the source and drainelectrodes 271 a to 271 c as masks through the channel protective layer280 and the buffer layer 262. By the addition of the impurity element,the impurity region 281 is selectively formed in the channel formationregion of the microcrystalline semiconductor film 261. Since the channelformation region of the microcrystalline semiconductor film 261 iscovered with the channel protective layer 280, the impurity region 281that is a channel doping region is selectively formed in the channelformation region. The impurity element can be added (introduced) by anion implantation method or an ion doping method.

In this embodiment mode, since the addition of the impurity element tothe microcrystalline semiconductor film 261 is performed through thechannel protective layer 280 and the buffer layer 262, damage (e.g.,surface roughness) to the microcrystalline semiconductor film 261 by theaddition can be reduced.

Through the above process, the channel-top-type thin film transistor 274is formed (see FIG. 6C). The insulating film 276 is formed so as tocover the thin film transistor 274, and a contact hole which reaches thesource or drain electrode 271 c is formed. The pixel electrode 277 isformed in the contact hole, so that the thin film transistor 274 and thepixel electrode 277 are electrically connected to each other (see FIG.6D).

Further, a shape in which the end portions of a source and drain regionsand the end portions of source and drain electrodes are aligned witheach other and successive may also be employed. A channel-stop-type thinfilm transistor 279 in which the end portions of a source and drainregions and the end portions of source and drain electrodes are alignedwith each other and successive is shown in FIG. 15. By performingetching of the source and drain electrodes and the source and drainregions with dry etching, the thin film transistor 279 with theabove-described shape can be obtained. Further, by etching asemiconductor film to which an impurity which imparts one conductivitytype is added, with the source and drain electrodes as masks to form thesource and drain regions, the thin film transistor 279 with theabove-described shape can also be obtained.

By forming a channel-stop-type thin film transistor as a thin filmtransistor, reliability of the thin film transistor can be improved.Further, by forming a channel formation region with the use of amicrocrystalline semiconductor film, a field-effect mobility of 1 to 20cm²/V·sec can be achieved. Accordingly, this thin film transistor can beused as a switching element of a pixel in a pixel portion and as anelement included in a scanning line (gate line) driver circuit.

According to this embodiment mode, a display device having a thin filmtransistor with excellent electric characteristics and high reliabilitycan be manufactured.

(Embodiment Mode 3)

An example of a manufacturing process in which a microcrystallinesemiconductor film is irradiated with laser light will be described inthis embodiment mode.

A gate electrode is formed over a substrate, and a gate insulating filmis formed so as to cover the gate electrode. Then, a microcrystallinesilicon (SAS) film is deposited as a microcrystalline semiconductor filmover the gate insulating film. The thickness of the microcrystallinesemiconductor film is greater than or equal to 1 nm and less than 15 nm,preferably, 2 to 10 nm inclusive. In particular, the microcrystallinesemiconductor film with a thickness of 5 nm (4 to 8 am) has highabsorptance of laser light, and thus, improves productivity.

In the case where the microcrystalline semiconductor film is formed overthe gate insulating film by a plasma CVD method or the like, a regionwhich contains more amorphous components than a semiconductor film whichcontains crystals (here such a region is referred to as an interfaceregion) is formed near the interface between the gate insulating filmand the semiconductor film which contains crystals, in some cases.Further, in the case where an ultra-thin microcrystalline semiconductorfilm with a thickness of about less than or equal to 10 nm is formed bya plasma CVD method or the like, a semiconductor film which containsmicrocrystal grains can be formed, but it is difficult to obtain asemiconductor film which contains microcrystal grains uniformly withhigh quality throughout the film, in these cases, a laser process forirradiation with laser light which is described below is effective.

Next, the microcrystalline semiconductor film is irradiated with laserlight from the surface side. The irradiation with the laser light isperformed with the energy density as low as the microcrystallinesemiconductor film is not melted. That is, the laser process(hereinafter also referred to as ‘LP’) of this embodiment mode iscarried out by solid-phase crystal growth which is performed byradiation heating without melting the microcrystalline silicon film.That is, the process utilizes a critical region where a depositedmicrocrystalline silicon film is not brought into a liquid phase, and inthat sense, the process can also be referred to as ‘critical growth’.

The laser light can affect a region to the interface between themicrocrystalline silicon film and the gate insulating film. Accordingly,using crystals on the surface side of the microcrystalline silicon filmas seeds, solid-phase crystal growth proceeds from the surface towardthe interface with the gate insulating film, and roughly columnarcrystals grow. Solid-phase crystal growth by an LP process does notincrease a crystal diameter but improves the crystallinity in afilm-thickness direction.

In the LP process, for example, a microcrystalline silicon film over aglass substrate of 730 mm×920 mm can be processed with one laser beamscan, by collecting a laser beam into a long rectangular shape (a linearlaser beam). In this case, the proportion of overlap of linear laserbeams (the overlap rate) is set to 0 to 90% (preferably, 0 to 67%).Accordingly, process time for each substrate can be shortened, andproductivity can be increased. The shape of the laser beam is notlimited to a linear shape, and similar processing can be conducted usinga planar laser beam. Further, the LP process of this embodiment mode isnot limited to be used for the glass substrate with the above-describedsize and can be used for substrates with various sizes.

The LP process has effects in improving crystallinity of an interfaceregion with the gate insulating film and improving electriccharacteristics of a thin film transistor having a bottom-gate structurelike the thin film transistor of this embodiment mode.

Such critical growth also has a feature in that roughness (a projectioncalled a ridge) of a surface, which is observed in the case ofconventional low-temperature polysilicon, is not formed and thesmoothness of a silicon surface is maintained even after the LP process.

A crystalline silicon film which is obtained by the action of the laserbeam directly on the microcrystalline silicon film after the formationas in this embodiment mode is distinctly different in growth mechanismand film quality from a conventional microcrystalline silicon film whichis obtained by being just deposited and a microcrystalline silicon filmwhich is modified by conduction heating (the one disclosed in Reference1). In this specification, a crystalline semiconductor film which isobtained through the LP process performed to a microcrystallinesemiconductor film after being deposited is referred to as an LPSASfilm.

After the microcrystalline semiconductor film such as an LPSAS film isformed, an amorphous silicon (a-Si:H) film is formed as a buffer layerby a plasma CVD method at 300° C. to 400° C. By the formation of theamorphous silicon film, hydrogen is supplied to the LPSAS film, and thesame effect as hydrogenation of the LPSAS film can be achieved. That is,by deposition of the amorphous silicon film over the LPSAS film,hydrogen is diffused into the LPSAS film, so that a dangling bond can beterminated.

Subsequent steps arc similar to those in Embodiment Mode 1, so that achannel protective layer is formed, and a mask is formed thereover.Next, the microcrystalline semiconductor film and the buffer layer areetched using the mask. Then, a semiconductor film to which an impuritywhich imparts one conductivity type is added and a conductive film areformed, and a mask is formed over the conductive film. Next, using themask, the conductive film is etched to be divided to form a source anddrain electrodes. Further, using the same mask, the semiconductor filmto which an impurity which imparts one conductivity type is added isetched using the channel protective layer as an etching stopper, so thata source and drain regions are formed. Furthermore, using the same maskor the source and drain electrodes as a mask or masks, an impurityelement which imparts one conductivity type is added to themicrocrystalline semiconductor film through the channel protectivelayer, so that an impurity region that is a channel doping region isformed selectively in the channel formation region of themicrocrystalline semiconductor film.

Through the above-described process, a channel-stop-type thin filmtransistor can be formed, and a display device having thechannel-stop-type thin film transistor can be manufactured.

Further, this embodiment mode can be combined with Embodiment Mode 1 or2 as appropriate.

(Embodiment Mode 4)

An example in which the shape of a thin film transistor is different inEmbodiment Mode 1 will be described in this embodiment mode. Except theshape, the thin film transistor can be formed in a similar manner toEmbodiment Mode 1; thus, repetitive description of the same componentsas or components having similar functions to Embodiment Mode 1 andmanufacturing steps for forming those components will be omitted.

A channel-stop-type bottom-gate thin film transistor 874 of thisembodiment mode is shown in FIG. 31.

In FIG. 31, the channel-stop-type thin film transistor 874 is providedover a substrate 850. The channel-stop-type thin film transistor 874includes a gate electrode 851, gate insulating films 852 a and 852 b, amicrocrystalline semiconductor film 861, a channel protective layer 880,a buffer layer 862, a source and drain regions 872, and source and drainelectrodes 871 a, 871 b, and 871 c. An insulating film 876 is providedso as to cover the thin film transistor 874. A pixel electrode 877 isformed so as to be in contact with the source or drain electrode 871 cin a contact hole formed in the insulating film 876.

Furthermore, in a channel formation region of the microcrystallinesemiconductor film 861, an impurity region 883 containing an impurityelement of one conductivity type is provided selectively in a regionwhich is not overlapped with the source and drain electrodes 871 a, 871b, and 871 c.

Further, in this embodiment mode, the channel protective layer 880 isformed in contact with the channel formation region of themicrocrystalline semiconductor film 861, and the buffer layer 862 isformed over the channel protective layer 880 and the microcrystallinesemiconductor film 861. In this embodiment mode, the impurity region 883is irradiated with light after an impurity element is added, so that aneffect by channel doping is further improved.

This embodiment mode is one feature of the present invention and astructure in which a channel protective layer is formed in contact witha channel formation region of a microcrystalline semiconductor film anda buffer layer is formed over the channel protective layer and themicrocrystalline semiconductor film. In this case, an impurity regionwhich is selectively formed in the channel formation region of themicrocrystalline semiconductor film can be irradiated with laser lightthrough the channel protective layer. Activation and improvement ofcrystallinity of the impurity region can be performed by the laser lightirradiation, so that the effect of channel doping can be improved.Further, the channel protective layer functions as an antireflectionfilm with respect to laser light, so that more efficient laserirradiation can be performed on the microcrystalline semiconductor film.

A method for manufacturing a display device having the thin filmtransistor of this embodiment mode shown in FIG. 31 will be describedusing FIGS. 32A to 32C and FIGS. 33A to 33C. The gate electrode 851 isformed over the substrate 850, the gate insulating films 852 a and 852 bare formed over the gate electrode 851, and the microcrystallinesemiconductor film 861 is formed thereover. The channel protective layer880 is formed in a region which is overlapped with a channel formationregion of the microcrystalline semiconductor film 861 (see FIG. 32A). Inthis embodiment mode, the channel protective layer 880 is formed on themicrocrystalline semiconductor film 861 so as to be in contact with eachother.

The laser light irradiation process described in Embodiment Mode 3 maybe performed on the microcrystalline semiconductor film 861.

Over the microcrystalline semiconductor film 861 and the channelprotective layer 880, a buffer layer 854, a semiconductor film 863 towhich an impurity which imparts one conductivity type is added, andconductive films 865 a, 865 b, and 865 c are stacked (see FIG. 32B).

With the use of a mask 866, the buffer layer 854, the semiconductor film863 to which an impurity which imparts one conductivity type is added,and the conductive films 865 a, 865 b, and 865 c are etched, so that thebuffer layer 862, the source and drain regions 872, and the source anddrain electrodes 871 a, 871 b, and 971 c are formed. In this embodimentmode, the conductive films 865 a, 865 b, and 865 c are etched by wetetching, and the buffer layer 854 and the semiconductor film 863 towhich an impurity which imparts one conductivity type is added areetched by dry etching.

An example in which the buffer layer, the source and drain electrodes,and the semiconductor film to which an impurity which imparts oneconductivity type is added are etched by the same step is described inthis embodiment mode. Therefore, the buffer layer, the semiconductorfilm to which an impurity which imparts one conductivity type is added,and the source and drain electrodes are formed while reflecting almostthe same shape. If they are etched by one etching process as describedabove, the manufacturing process can be simplified, and the number ofmasks used in the etching process can be reduced.

An impurity element 882 which imparts one conductivity type is added tothe microcrystalline semiconductor film 861 with the mask 866 throughthe channel protective layer 880 which is exposed between the source anddrain electrodes 871 a to 871 c. By the addition of the impurity element882, an impurity region 881 is selectively formed in the channelformation region of the microcrystalline semiconductor film 861. Sincethe channel formation region of the microcrystalline semiconductor film861 is covered with the channel protective layer 880, the impurityregion 881 that is a channel doping region is selectively formed in thechannel formation region. The impurity element 882 can be added(introduced) by an ion implantation method or an ion doping method.

In this embodiment mode, since the addition of the impurity element tothe microcrystalline semiconductor film 861 is performed through thechannel protective layer 880, damage (e.g., surface roughness) to themicrocrystalline semiconductor film 861 by the addition can be reduced.

Control of the threshold voltage by a channel doping method is carriedout by the concentration of an impurity element. In this embodimentmode, channel doping is performed not on the whole of the channelformation region but so as to selectively form a channel doping region.Therefore, in the present invention, the threshold voltage can becontrolled more precisely by controlling the area of the channelformation region. In the case where the impurity is added to themicrocrystalline semiconductor film through the channel protectivelayer, it is difficult to control the concentration of the impurityelement in the microcrystalline semiconductor film which exists deep ina film-thickness direction and the control tends to be varied, and thereis a fear of damage to the film because the addition needs to beperformed at energy high enough to pass the impurity element through thechannel protective layer. According to the present invention, filmdamage to the microcrystalline semiconductor film can be prevented andthe threshold value can be controlled more accurately and uniformly.Accordingly, high reliability and high performance can be achieved in athin film transistor and a display device including the thin filmtransistor.

The microcrystalline semiconductor film has low n-type conductivity whenan impurity element for controlling valence electrons is not addedthereto intentionally. Therefore, by addition of an impurity elementwhich imparts p-type conductivity to the microcrystalline semiconductorfilm which functions as a channel formation region of a thin filmtransistor, the threshold voltage can be controlled. A typical exampleof the impurity element which imparts p-type conductivity is boron; andan impurity gas such as B₂H₆ or BF₃ may be added to silicon hydride at 1ppm to 1000 ppm, preferably, 1 ppm to 100 ppm. The concentration ofboron may be set to 1×10¹⁴ to 6×10¹⁶ atoms/cm³.

Further, in this embodiment mode, the channel protective layer 880 isformed in contact with the channel formation region of themicrocrystalline semiconductor film 861 and the buffer layer 862 isformed over the channel protective layer 880 and the microcrystallinesemiconductor film 861. In this case, the impurity region 881 which isselectively formed in the channel formation region of themicrocrystalline semiconductor film 861 can be irradiated with laserlight 884 through the channel protective layer. The impurity region 881can be reformed to be the impurity region 883 by the irradiation withthe laser light 884 (see FIG. 3B). Activation and improvement ofcrystallinity of the impurity region can be performed by the laser lightirradiation, so that the effect of channel doping can be improved.Further, the channel protective layer 880 functions as an antireflectionfilm against the laser light 884, so that more efficient laserirradiation can be performed on the microcrystalline semiconductor film861. Further, the channel protective layer 880 functions also as aprotective layer, so that damage to the microcrystalline semiconductorfilm 861 due to laser irradiation, such as surface roughness ordeformation of a film can be prevented.

Irradiation conditions (e.g., light energy, wavelength, and irradiationperiod of time) of light irradiation of an impurity region that is achannel doping region of a microcrystalline semiconductor film may beset as appropriate in accordance with a material or a thickness of achannel protective layer through which light passes, a material,thickness, and the like of the microcrystalline semiconductor film.

As a laser for emitting the laser light 884, a continuous wave laser, apseudo continuous wave laser, or a pulsed laser can be used. Forexample, a gas laser such as an excimer laser such as a KrF laser, and agas laser such as art Ar laser, or a Kr laser can be given. Further, asolid-state laser such as a YAG laser, a YVO₄ laser, a YLF laser, aYAlO₃ laser, a GdVO₄ laser, a KGW laser, a KYW laser, an alexandritelaser, a Ti:sapphire laser, a Y₂O₃ laser, or the like can be given. Notethat an excimer laser is a pulsed laser, and some solid-state laserssuch as a YAG laser can be used as any of a continuous wave laser, apseudo continuous wave laser, and a pulsed laser. Note that, in asolid-state laser, the second to fifth harmonics can be preferably used.Further alternatively, a semiconductor laser such as a GaN laser, a GaAslaser, a GaAlAs laser, or an InGaAsP laser can be used.

Further, lamp light may be used as well. For example, light emitted froman ultraviolet ray lamp, a black-light, a halogen lamp, a metal halidelamp, a xenon arc lamp, a carbon arc lamp, a high-pressure sodium vaporlamp, or a high-pressure mercury vapor lamp may be used.

Through the above-described process, the channel-stop-type thin filmtransistor 874 is formed (see FIG. 33C). The insulating film 876 isformed so as to cover the thin film transistor 874, and the contact holewhich reaches the source or drain electrode 871 c is formed. The pixelelectrode 877 is formed in the contact hole, so that the thin filmtransistor 874 and the pixel electrode 877 are electrically connected toeach other (see FIG. 31).

Further, a shape in which the end portions of a buffer layer, a sourceand drain regions, and source and drain electrodes are aligned with eachother may also be employed. A thin film transistor in which the endportions of a buffer layer, a source and drain regions, and source anddrain electrodes are aligned with each other can be formed as follows:etching of source and drain electrodes and etching of a buffer layer anda source and drain regions are performed with dry etching; or etching ofa buffer layer and a semiconductor film to which an impurity whichimparts one conductivity type is added, is performed with source anddrain electrodes as masks to form the buffer layer and the source anddrain regions.

By forming a channel-stop-type thin film transistor as a thin filmtransistor, reliability of the thin film transistor can be improved.Further, by forming a channel formation region with the use of amicrocrystalline semiconductor film, a field-effect mobility of 1 to 20cm²/V·sec can be achieved. Accordingly, this thin film transistor can beused as a switching element of a pixel in a pixel portion and as anelement included in a scanning line (gate line) driver circuit.

According to this embodiment mode, a display device having a thin filmtransistor with high electric characteristics and high reliability canbe manufactured.

(Embodiment Mode 5)

An example of the manufacturing process of the display device describedin any of Embodiment Modes 1 to 4 will be described in detail in thisembodiment mode. Therefore, repetitive description of the samecomponents as or components having similar functions to Embodiment Modes1 to 4 and manufacturing steps for forming those components will beomitted.

In Embodiment Modes 1 to 4, before the microcrystalline semiconductorfilm is formed, a reaction chamber may be subjected to cleaning and/orflush (washing) treatment (e.g., hydrogen flush using hydrogen as aflush substance or silane flush using silane as a flush substance). Bythe flush treatment, a film to be formed can be prevented from beingcontaminated by an impurity such as oxygen, nitrogen, or fluorine in thereaction chamber.

The flushing treatment can remove impurities such as oxygen, nitrogen,and fluorine in the reaction chamber. For example, flush treatment isperformed in the following manner: a plasma CVD apparatus is used, andmonosilane is used as a flush substance and introduced into a chamber ata gas flow rate of 8 to 10 SLM for 5 to 20 minutes, preferably, 10 to 15minutes to perform silane flush treatment. Note that 1 SLM is 1000 sccm,that is, 0.06 m³/h.

Cleaning can be performed with the use of, for example, fluorineradicals. Note that the inside of the reaction chamber can be cleaned byintroduction of fluorine radicals into the reaction chamber, which aregenerated by introduction of carbon fluoride, nitrogen fluoride, orfluorine into a plasma generator provided outside of the reactionchamber and by dissociation thereof.

Flush treatment may also be performed before the gate insulating film,the buffer layer, the channel protective layer, and the semiconductorfilm to which an impurity which imparts one conductivity type is addedare formed. Note that it is effective that flush treatment is performedafter cleaning.

Before a substrate is carried into a reaction chamber to perform filmformation, the inner wall of each reaction chamber may be coated with aprotective film that is a film to be formed (this coating is alsoreferred to as pre-coating treatment). Pre-coating treatment istreatment in which plasma treatment is performed by flowing of adeposition gas in a reaction chamber to coat the inner wall of thereaction chamber with a thin protective film in advance. For example,before a microcrystalline silicon film is formed as the microcrystallinesemiconductor film, pre-coating treatment may be performed in which theinner wall of the reaction chamber is coated with an amorphous siliconfilm with a thickness of 0.2 to 0.4 μm. Flush treatment (hydrogen flush,silane flush, or the like) may also be performed after pre-coatingtreatment. In the case of performing cleaning and/or pre-coatingtreatment, it is necessary that a substrate is carried out from areaction chamber. However, in the case of performing flush treatment(hydrogen flush, silane flush, or the like), a substrate may be kept ina reaction chamber because plasma treatment is not performed.

A protective film formed of an amorphous silicon film is formed in areaction chamber in which a microcrystalline silicon film is to beformed, and hydrogen plasma treatment is performed before filmformation, whereby the protective film is etched and an extremely smallamount of silicon is deposited on a substrate and can be a nucleus ofcrystal growth.

By the pre-coating treatment, a film to be formed can be prevented frombeing contaminated by an impurity such as oxygen, nitrogen, or fluorinein the reaction chamber.

Pre-coating treatment may also be performed before formation of a gateinsulating film and a semiconductor film to which an impurity whichimparts one conductivity type is added.

Furthermore, an example of a method for forming a gate insulating film,a microcrystalline semiconductor film, and a buffer layer will bedescribed in detail.

Examples of a plasma CVD apparatus which can be used in the presentinvention will be described using FIGS. 13A and 13B. FIGS. 13A and 13Beach show a microwave plasma CVD apparatus which can perform successivefilm formation. FIGS. 13A and 13B are plane views each schematicallyshowing an upper cross-section of a microwave plasma CVD apparatus. Aloading chamber 1110, an unloading chamber 1115, and reaction chambers(1) to (4), 1111 to 1114, are provided around a common chamber 1120.Gate valves 1122 to 1127 are provided between the common chamber 1120and the chambers so that treatment in each chamber does not haveinfluence on treatment in other chambers. Note that the number ofreaction chambers is not limited to four; and the number of reactionchambers may be more than four or less than four. When the number ofreaction chambers is large, reaction chambers can be allocated accordingto the kind of a film to be stacked; thus, the number of cleaning of thereaction chamber can be reduced. FIG. 13A shows an example of amicrowave plasma CVD apparatus having four reaction chambers, and FIG.13B shows an example of a microwave plasma CVD apparatus having threereaction chambers.

An example will be described in which a gate insulating layer, amicrocrystalline semiconductor film, a buffer layer, and a channelprotective layer are formed using the plasma CVD apparatus shown in FIG.13A. Substrates are loaded into a cassette 1128 in the loading chamber1110 and a cassette 1129 in the unloading chamber 1115 and carried tothe reaction chambers (1) to (4), 111 to 1114, with a transport means1121 of the common chamber 1120. In this apparatus, a reaction chambercan be allocated for each of different kinds of deposition films, sothat a plurality of different films can be formed successively withoutexposure to the air. Further, the reaction chamber may also be used as areaction chamber for performing an etching process or a laserirradiation process, as well as a film-formation process. By providingreaction chambers for various processes, various processes can beperformed without exposure to the air.

In each of the reaction chambers (1) to (4), the gate insulating film,the microcrystalline semiconductor film, the buffer layer, and thechannel protective layer are stacked. In this case, different kinds offilms can be stacked successively by switching a material gas. In thiscase, after the gate insulating films are formed, silicon hydride suchas silane is introduced into the reaction chamber, so that residualoxygen and silicon hydride are reacted with each other, and the reactantis exhausted from the reaction chamber, whereby the concentration ofresidual oxygen in the reaction chamber can be decreased. Accordingly,the concentration of oxygen to be contained in the microcrystallinesemiconductor film can be decreased. Further, crystal grains in themicrocrystalline semiconductor film can be prevented from beingoxidized.

Further, in a plasma CVD apparatus, films of one kind may be formed in aplurality of reaction chambers in order to improve productivity. Iffilms of one kind can be formed in a plurality of reaction chambers,films can be formed over a plurality of substrates at the same time. Forexample, in FIG. 13A, the reaction chambers (1) and (2) are used asreaction chambers in each of which a microcrystalline semiconductor filmis formed, the reaction chamber (3) is used as a reaction chamber inwhich an amorphous semiconductor film is formed, and the reactionchamber (4) is used as a reaction chamber in which a channel protectivelayer is formed. In the case where a plurality of substrates is treatedat the same time, as described above, a plurality of reaction chambersmay be provided, in each of which a film whose deposition rate is low isformed, so that productivity can be improved.

Before a substrate is carried into a reaction chamber to perform filmformation, cleaning and/or flush (washing) treatment (hydrogen flush,silane flush, or the like) are/is preferably performed, and the innerwall of each reaction chamber is preferably coated with a protectivefilm that is a film to be formed (this coating is also referred to aspre-coating treatment). Pre-coating treatment is treatment in whichplasma treatment is performed by flowing of a deposition gas in areaction chamber to coat the inner wall of the reaction chamber with athin protective film in advance. For example, before a microcrystallinesilicon film is formed as the microcrystalline semiconductor film,pre-coating treatment may be performed in which the inner wall of thereaction chamber is coated with an amorphous silicon film with athickness of 0.2 to 0.4 Flush treatment (hydrogen flush, silane flush,or the like) may also be performed after pre-coating treatment. In thecase of performing cleaning and/or pre-coating treatment, it isnecessary that a substrate is carried out from a reaction chamber.However, in the case of performing flush treatment (hydrogen flush,silane flush, or the like), a substrate may be kept in a reactionchamber because plasma treatment is not performed.

A protective film formed of an amorphous silicon film is formed on theinner wall of a reaction chamber in which a microcrystalline siliconfilm is to be formed, and hydrogen plasma treatment is performed beforefilm formation, whereby the protective film is etched and an extremelysmall amount of silicon is deposited on a substrate and can be a nucleusof crystal growth.

In this manner, with the microwave plasma CVD apparatus in which aplurality of chambers is connected, the gate insulating film, themicrocrystalline semiconductor film, the buffer layer, the channelprotective layer, and the semiconductor film to which an impurity whichimparts one conductivity type is added can be formed at the same time;thus, the mass productivity can be enhanced. Further, even whenmaintenance or cleaning is performed in one of reaction chambers, filmdeposition can be performed in the other reaction chambers, whereby tacttime for film formation can be shortened. Furthermore, each interfacebetween stacked layers can be formed without being contaminated by anair constituent or a contaminant impurity element floating in the air.Thus, variations in characteristics of thin film transistors can bereduced.

With the use of a microwave plasma CVD apparatus having such astructure, films of similar kinds or films of one kind can be formed ineach reaction chamber and can be formed successively without exposure tothe air. Therefore, each interface between stacked layers can be formedwithout being contaminated by a residue of a previously formed film oran impurity element floating in the air.

Further, a microwave generator and a high frequency wave generator maybe provided; thus, the gate insulating film, the microcrystallinesemiconductor film, the channel protective layer, and the semiconductorfilm to which an impurity which imparts one conductivity type is addedmay be formed by the microwave plasma CVD method, and the buffer layermay be formed by the high frequency plasma CVD method.

Note that, although the microwave plasma CVD apparatuses in FIGS. 13Aand 13B are each provided with the loading chamber and the unloadingchamber separately, a loading chamber and an unloading chamber may becombined to provide a loading and unloading chamber. Further, a sparechamber may be provided for the microwave plasma CVD apparatus. Bypre-heating a substrate in the spare chamber, it is possible to shortenheating time before film deposition in each reaction chamber, so thatthe throughput can be improved. In the film-formation treatment, a gassupplied from a gas supply portion may be selected in accordance withits purpose.

This embodiment mode can be implemented as appropriate in combinationwith any of the structures described in the other embodiment modes.

(Embodiment Mode 6)

Next, a process for manufacturing a display device will be describedusing FIGS. 10A and 10B and FIGS. 11A to 11C. As a display elementincluded in a display device, a light-emitting element utilizingelectroluminescence will be described in this embodiment mode.Light-emitting elements that use electroluminescence are differentiatedby whether a light-emitting material is an organic compound or aninorganic compound; generally, light-emitting elements using an organiccompound as a light-emitting material are called organic EL elements,and light-emitting elements using an inorganic compound as alight-emitting material are called inorganic EL elements. Further, thinfilm transistors 85 and 86 used in a display device are thin filmtransistors which have high electric characteristics and highreliability and can be manufactured in a similar manner to the thin filmtransistor 74 described in Embodiment Mode 1. Alternatively, the thinfilm transistor 274 or 874 described in Embodiment Mode 2 or 4 can beemployed as each of the thin film transistors 85 and 86.

In an organic EL element, by application of a voltage to alight-emitting element, electrons and holes are separately injected froma pair of electrodes into a layer containing a light-emitting organiccompound, and current flows. Then, by recombination of these carriers(electrons and holes), the organic compound with a light-emittingproperty forms an excited state, and light is emitted when the excitedstate returns to a ground state. Due to such mechanism, such alight-emitting element is called a current-excitation light-emittingelement.

Inorganic EL elements are classified depending on an element structureinto dispersion type inorganic EL elements and thin-film inorganic ELelements. A dispersion type inorganic EL element has a light-emittinglayer where particles of a light-emitting material are dispersed in abinder, and its light emission mechanism is donor-acceptor recombinationtype light emission that utilizes a donor level and an acceptor level.In a thin-film inorganic EL element, a light-emitting layer issandwiched between dielectric layers, and the dielectric layers aresandwiched between electrodes. Its light emission mechanism islocalized-type light emission that utilizes inner shell electronictransition of metal ions. Note that description will be made using anorganic EL element as a light-emitting element in this embodiment mode.Further, the channel-stop-type thin film transistor shown in FIG. 1 inEmbodiment Mode 1 is used as a thin film transistor for controllingdriving of a light-emitting element in this embodiment mode.

Through a similar process to that shown in FIG. 1, FIGS. 2A to 2E, FIGS.3A to 3C, and FIGS. 4A to 4D, the thin film transistors 85 and 86 areformed over a substrate 100, and an insulating film 87 which functionsas a protective film is formed over the thin film transistors 85 and 86as shown in FIGS. 10A and 10B. Next, a planarizing film 111 is formedover the insulating film 87, and a pixel electrode 112 connected to asource or drain electrode of the thin film transistor 86 is formed overthe planarizing film 111.

It is preferable that the planarizing film 111 be formed of an organicresin such as acrylic, polyimide, or polyamide, or siloxane.

In FIG. 10A, the thin film transistor of a pixel is an n-channel thinfilm transistor; thus, it is preferable that a cathode be used as thepixel electrode 112. To the contrary, it is preferable that an anode beused as the pixel electrode 112 in the case of a p-channel thin filmtransistor. Specifically, as the cathode, a material with low workfunction, such as Ca, Al, CaF, MgAg, or AlLi can be used.

Next, as shown in FIG. 10B, a bank 113 is formed over the planarizingfilm 111 and an end portion of the pixel electrode 112. The bank 113 hasan opening, in which the pixel electrode 112 is exposed. The bank 113 isformed of an organic resin film, an inorganic insulating film, or anorganic polysiloxane film. In particular, it is preferable that the bank113 be formed of a photosensitive material, and the opening be formedover the pixel electrode, and a side wall of the opening form aninclined surface with a continuous curvature.

Next, a light-emitting layer 114 is formed so as to be in contact withthe pixel electrode 112 in the opening of the bank 113. Thelight-emitting layer 114 may be formed of either a single layer or astacked layer of a plurality of layers.

Then, a common electrode 115 using an anode is formed to cover thelight-emitting layer 114. The common electrode 115 can be formed of alight-transmitting conductive film using any of the light-transmittingconductive materials for the pixel electrode 77 listed in EmbodimentMode 1. The common electrode 115 may also be formed of a titaniumnitride film or a titanium film as well as the above-describedlight-transmitting conductive film. In FIG. 10B, ITO is used for thecommon electrode 115. In the opening of the bank 113, a light-emittingelement 117 is formed by overlapping of the pixel electrode 112, thelight-emitting layer 114, and the common electrode 115. After that, aprotective film 116 is preferably formed over the common electrode 115and the bank 113 so that oxygen, hydrogen, moisture, carbon dioxide, orthe like does not enter the light-emitting element 117. As theprotective film 116, a silicon nitride film, a silicon nitride oxidefilm, a DLC film, or the like can be formed.

Furthermore, practically, after the steps up to the step of FIG. 10B arecompleted, it is preferable that packaging (sealing) be performed with aprotective film (e.g., an attachment film or an ultraviolet curableresin film) which has high airtightness and causes less degasification,or a covering material, in order to prevent further exposure to externalair.

Next, structures of light-emitting elements will be described usingFIGS. 11A to 11C. A cross-sectional structure of a pixel will bedescribed by taking an n-channel driving TFT as an example. Driving TFTs7001, 7011, and 7021 used for display devices shown in FIGS. 11A to 11Chave high electric characteristics and high reliability, and can bemanufactured in a similar manner to the thin film transistor 74described in Embodiment Mode 1. Alternatively, the thin film transistor274 or 874 described in Embodiment Mode 2 or 4 can be employed as eachof the MS 7001, 7011, and 7021.

In order to extract light emission, at least one of an anode and acathode of a light-emitting element is needed to be transparent. Thereare light-emitting elements having a top emission structure in whichlight emission is extracted through the surface opposite to thesubstrate, having a bottom emission structure in which light emission isextracted through the surface on the substrate side, and having a dualemission structure in which light emission is extracted through thesurface opposite to the substrate and the surface on the substrate side.A pixel structure of the present invention can be applied to alight-emitting element having any of the emission structures.

A light-emitting element having a top emission structure will bedescribed using FIG. 11A.

FIG. 11A is a cross-sectional view of a pixel in the case where thedriving TFT 7001 is an n-channel type and light is emitted from alight-emitting element 7002 to an anode 7005 side. In FIG. 11A, acathode 7003 of the light-emitting element 7002 is electricallyconnected to the driving TFT 7001, and a light-emitting layer 7004 andthe anode 7005 are stacked in this order over the cathode 7003. Thecathode 7003 can be formed of various conductive materials as long as itis a conductive film whose work function is low and which reflectslight. For example, Ca, Al, CaF, MgAg, AlLi, or the like is preferablyused. Further, the light-emitting layer 7004 may be formed of either asingle layer or a stacked layer of a plurality of layers. In the casewhere the light-emitting layer 7004 is formed of a plurality of layers,the light-emitting layer 7004 is formed by stacking anelectron-injecting layer, an electron-transporting layer, alight-emitting layer, a hole-transporting layer, and a hole-injectinglayer in this order over the cathode 7003. Note that not all the layersare needed to be provided. The anode 7005 is formed of alight-transmitting conductive film such as a film of indium oxideincluding tungsten oxide, indium zinc oxide including tungsten oxide,indium oxide including titanium oxide, indium tin oxide includingtitanium oxide, indium tin oxide (hereinafter, referred to as ITO),indium zinc oxide, or indium tin oxide to which silicon oxide is added.

A region where the light-emitting layer 7004 is sandwiched between thecathode 7003 and the anode 7005 corresponds to the light-emittingelement 7002. In the case of the pixel shown in FIG. 11A, light isemitted from the light-emitting element 7002 to the anode 7005 side asindicated by an arrow.

Next, a light-emitting element having a bottom emission structure willbe described using FIG. 11B. FIG. 11B is a cross-sectional view of apixel in the case where a driving TFT 7011 is an n-channel type, andlight is emitted from a light-emitting element 7012 to a cathode 7013side. In FIG. 11B, the cathode 7013 of the light-emitting element 7012is formed over a light-transmitting conductive film 7017 which iselectrically connected to the driving TFT 7011, and a light-emittinglayer 7014 and an anode 7015 are stacked in this order over the cathode7013. Note that a shielding film 7016 for reflecting or blocking lightmay be formed so as to cover the anode 7015 when the anode 7015 has alight-transmitting property. For the cathode 7013, various materials canbe used as in the case of FIG. 11A as long as they are conductivematerials whose work function is low. It is to be noted that thethickness of the cathode 7013 is set such that light is transmittedtherethrough (preferably, about 5 to 30 nm). For example, an aluminumfilm with a thickness of 20 nm can be used as the cathode 7013. Then, asin the case of FIG. 11A, the light-emitting layer 7014 may be formed ofeither a single layer or a stacked layer of a plurality of layers.Although the anode 7015 does not need to be able to transmit light,similarly to FIG. 11A, it can be formed of a light-transmittingconductive material. As the shielding film 7016, a metal or the likethat reflects light can be used; however, it is not limited to a metalfilm. For example, a resin or the like to which a black pigment is addedcan be used.

A region where the light-emitting layer 7014 is sandwiched between thecathode 7013 and the anode 7015 corresponds to the light-emittingelement 7012. In the case of the pixel shown in FIG. 11B, light isemitted from the light-emitting element 7012 to the cathode 7013 side asindicated by an arrow.

Next, a light-emitting element having a dual emission structure will bedescribed using FIG. 11C. In FIG. 11C, a cathode 7023 of alight-emitting element 7022 is formed over a light-transmittingconductive film 7027 which is electrically connected to the driving TFT7021, and a light-emitting layer 7024 and an anode 7025 are stacked inthis order over the cathode 7023. As in the case of FIG. 11A, thecathode 7023 can be formed of various conductive materials as long asthey are conductive materials whose work function is low. It is to benoted that the cathode 7023 has a thickness to transmit light. Forexample, Al having a film thickness of 20 nm can be used as the cathode7023. Then, as in FIG. 11A, the light-emitting layer 7024 may be formedof either a single layer or a stacked layer of a plurality of layers.The anode 7025 can be, as in FIG. 11A, formed of a light-transmittingconductive material.

A region where the cathode 7023, the light-emitting layer 7024, and theanode 7025 are overlapped with each other corresponds to thelight-emitting element 7022. In the case of the pixel shown in FIG. 11C,light is emitted from the light-emitting element 7022 to both the anode7025 side and the cathode 7023 side as indicated by arrows.

Note that, although an organic EL element is described as alight-emitting element in this embodiment mode, it is also possible toprovide an inorganic EL element as a light-emitting element.

Note that, although the example in which a thin film transistor (adriving TFT) for controlling driving of a light-emitting element iselectrically connected to the light-emitting element is described inthis embodiment mode, a TFT for controlling current may be connectedbetween the driving TFT and the light-emitting element.

Note that the display device described in this embodiment mode is notlimited to have any of the structures shown in FIGS. 11A to 11C, and canbe modified in various ways based on the technical idea of the presentinvention.

Through the above-described process, a light-emitting device can bemanufactured as a display device. Since a thin film transistor with highelectric characteristics and high reliability is used in thelight-emitting device of this embodiment mode, the light-emitting devicehas high contrast and high visibility.

This embodiment mode can be implemented as appropriate in combinationwith any of the structures described in the other embodiment modes.

(Embodiment Mode 7)

Examples of a liquid crystal display device including the thin filmtransistor described in any one of Embodiment Modes 1 to 5 will bedescribed in this embodiment mode using FIGS. 17 to 30. In the examplesof a liquid crystal display device described in this embodiment modeusing FIGS. 17 to 30, a liquid crystal display element is used as adisplay element. The thin film transistor described in Embodiment Mode1, 2, or 4 can be applied to each of a TFT 628 and a TFT 629 used forliquid crystal display devices shown in FIGS. 17 to 30, and the TFTs 628and 629 are thin film transistors with high electric characteristics andhigh reliability, which can be manufactured in a similar manner throughthe process described in Embodiment Modes 3 to 5. The TFT 628 and theTFT 629 include a channel protective layer 608 and a channel protectivelayer 611, respectively, and are inverted staggered thin filmtransistors each using a microcrystalline semiconductor film as achannel formation region. Further, the TFT 628 and the TFT 629 eachinclude an impurity region (a so-called channel-doping region)selectively in the channel formation region of the microcrystallinesemiconductor film. In each of the TFTs 628 and 629, the impurity regionis formed by selectively adding as an impurity which imparts oneconductivity type, boron that is an impurity element which impartsp-type conductivity, and the threshold voltage of the TFT is controlled.

First, a vertical alignment (VA) mode liquid crystal display device willbe described. The VA mode of a liquid crystal display device is a kindof mode in which alignment of liquid crystal molecules of a liquidcrystal display panel is controlled. According to the VA mode liquidcrystal display device, liquid crystal molecules are aligned to bevertical to a panel surface when voltage is not applied. In particular,in this embodiment mode, it is devised that a pixel is divided intoseveral regions (sub-pixels) so that molecules are aligned in differentdirections. This is referred to as multi-domain or multi-domain design.In the following description, a liquid crystal display device withmulti-domain design will be described.

FIGS. 18 and 19 show a pixel electrode and a counter electrode,respectively. Note that FIG. 18 is a plan view on a substrate side wherethe pixel electrode is formed. FIG. 17 shows a cross-sectional structurealong dashed line G-H in FIG. 18. FIG. 19 is a plan view on a substrateside where the counter electrode is formed. Hereinafter, descriptionwill be made with reference to these drawings.

FIG. 17 shows a state in which a substrate 600 provided with the TFT628, a pixel electrode 624 connected to the TFT 628, and a storagecapacitor 630 overlaps with a counter substrate 601 provided with acounter electrode 640 and the like, and liquid crystals are injected.

At the position where a spacer 642 is provided in the counter substrate601, a light shielding film 632, a first color film 634, a color film636, a third color film 638, and the counter electrode 640 are formed.With this structure, the height of a projection 644 for controllingalignment of liquid crystals is made different from that of the spacer642. An alignment film 648 is formed over the pixel electrode 624.Similarly, the counter electrode 640 is also provided with an alignmentfilm 646. A liquid crystal layer 650 is formed between the alignmentfilms 646 and 648.

Although a columnar spacer is used for the spacer 642 in this embodimentmode, bead spacers may be dispersed. Further, the spacer 642 may beformed over the pixel electrode 624 formed over the substrate 600.

The TFT 628, the pixel electrode 624 connected to the TFT 628, and thestorage capacitor 630 are formed over the substrate 600. The pixelelectrode 624 is connected to a wiring 618 in a contact hole 623 whichpenetrates an insulating film 620 which covers the TFT 628, the wiring618, and the storage capacitor 630 and also penetrates an insulatingfilm 622 which covers the insulating film 620. As the TFT 628, the thinfilm transistor described in Embodiment Mode 1 can be used asappropriate. Further, the storage capacitor 630 includes a firstcapacitor wiring 604 which is formed in a similar manner to a gatewiring 602 of the TFT 628, a gate insulating film 606, and a secondcapacitor wiring 617 which is formed in a similar manner to a wiring 616and the wiring 618. Further, in FIGS. 17 to 20, in the TFT 628, amicrocrystalline semiconductor film, a buffer layer, a semiconductorfilm to which an impurity which imparts one conductivity type is addedand which functions as a source and drain regions, and a wiring whichalso functions as a source and drain electrodes are patterned by thesame etching process and stacked with almost the same shape.

A liquid crystal element is formed by overlapping of the pixel electrode624, the liquid crystal layer 650, and the counter electrode 640.

FIG. 18 shows a structure over the substrate 600. The pixel electrode624 is formed of a material described in Embodiment Mode 1. The pixelelectrode 624 is provided with a slit 625. The slit 625 is forcontrolling alignment of liquid crystals.

The TFT 629, the pixel electrode 626 connected to the TFT 629, and thestorage capacitor 631 shown in FIG. 18 can be formed in a similar mannerto the TFT 628, the pixel electrode 624, and the storage capacitorportion 630, respectively. Both the TFTs 628 and 629 are connected tothe wiring 616. A pixel of this liquid crystal display panel includesthe pixel electrodes 624 and 626. Each of the pixel electrodes 624 and626 is a sub-pixel.

FIG. 19 shows a structure on a counter substrate side. The counterelectrode 640 is formed over the light shielding film 632. The counterelectrode 640 is preferably formed of a similar material to the pixelelectrode 624. The projection 644 which controls alignment of liquidcrystals is formed over the counter electrode 640. Further, the spacer642 is formed in accordance with the position of the light shieldingfilm 632.

FIG. 20 shows an equivalent circuit of this pixel structure. Both theTFTs 628 and 629 are connected to the gate wiring 602 and the wiring616. In this case, by making the potential of the capacitor wiring 604different from that of a capacitor wiring 605, operation of a liquidcrystal element 651 can be different from that of a liquid crystalelement 652. That is, alignment of the liquid crystals is preciselycontrolled and a viewing angle is increased by individual control ofpotentials of the capacitor wirings 604 and 605.

When voltage is applied to the pixel electrode 624 provided with theslit 625, electric field distortion (an oblique electric field) isgenerated near the slit 625. The slit 625 and the projection 644 on thecounter substrate 601 side are alternately arranged so as to be engagedwith each other, and an oblique electric field is effectively generatedto control alignment of liquid crystals, so that a direction ofalignment of the liquid crystals varies depending on the position. Thatis, a viewing angle of a liquid crystal panel is expanded bymulti-domain.

Next, a VA mode liquid crystal display device, which is different fromthe above-described device, will be described with reference to FIGS. 21to 24.

FIGS. 21 and 22 each show a pixel structure of a VA mode liquid crystaldisplay panel. FIG. 22 is a plan view of the substrate 600. FIG. 21shows a cross-sectional structure along dashed line Y-Z in FIG. 22. Thefollowing explanation will be made with reference to both the drawings.

In this pixel structure, one pixel has a plurality of pixel electrodes,and a TFT is connected to each pixel electrode. Each TFT is formed so asto be driven with a different gate signal. That is, in the pixel ofmulti-domain design, a signal applied to each pixel electrode iscontrolled independently.

In the contact hole 623, the pixel electrode 624 is connected to the TFT628 through the wiring 618. Further, in a contact hole 627, the pixelelectrode 626 is connected to the TFT 629 through a wiring 619. The gatewiring 602 of the TFT 628 and a gate wiring 603 of the TFT 629 areseparated so that different gate signals can be supplied. In contrast,the wiring 616 which serves as a data line is used in common for theTFTs 628 and 629. As each of the TFTs 628 and 629, the thin filmtransistor described in Embodiment Mode 1 can be used as appropriate.Further, a capacitor wiring 690 is provided. Further, in FIGS. 21 to 30,in the TFT 628 and the TFT 629, semiconductor films to which an impuritywhich imparts one conductivity type is added and which function assource and drain regions and wirings which also function as source anddrain electrodes are patterned by the same etching process and stackedwith almost the same shape.

The shape of the pixel electrode 624 is different from that of the pixelelectrode 626, and the pixel electrodes are separated by the slit 625.The pixel electrode 626 is formed so as to surround the external side ofthe pixel electrode 624 which is spread into a V shape. Timing ofvoltage application is made vary between the pixel electrodes 624 and626 by the TFTs 628 and 629 in order to control alignment of liquidcrystals. FIG. 24 shows an equivalent circuit of this pixel structure.The TFT 628 is connected to the gate wiring 602, and the TFT 629 isconnected to the gate wiring 603. By supplying different gate signals tothe gate wirings 602 and 603, operation timings of the TFTs 628 and 629can vary.

The counter substrate 601 is provided with the light shielding film 632,the color film 636, and the counter electrode 640. Further, aplanarizing film 637 is formed between the color film 636 and thecounter electrode 640 to prevent alignment disorder of the liquidcrystals. FIG. 23 shows a structure on the counter substrate side. Aslit 641 is formed in the counter electrode 640 which is used in commonbetween different pixels. This slit 641 is disposed so as to alternatelyengage with the slit 625 on the pixel electrodes 624 and 626 side,whereby an oblique electric field is generated effectively to controlalignment of the liquid crystals. Accordingly, the alignment of theliquid crystals can be varied depending on the portion, so that theviewing angle is widened.

The pixel electrode 624, the liquid crystal layer 650, and the counterelectrode 640 overlap with each other, so that a first liquid crystalelement is formed. Further, a second liquid crystal element is formed byoverlapping of the pixel electrode 626, the liquid crystal layer 650,and the counter electrode 640. Further, the structure is a multi-domainstructure in which the first liquid crystal element and the secondliquid crystal element are included in one pixel.

Next, a horizontal electric-field liquid crystal display device will bedescribed. The horizontal electric-field mode is a method in which anelectric field is horizontally applied to liquid crystal molecules in acell, whereby liquid crystals are driven to express a gray scale.According to this method, the viewing angle can be increased to about180 degrees. Hereinafter, a liquid crystal display device employing thehorizontal electric-field mode will be described.

FIG. 25 illustrates a state in which the substrate 600 provided with theTFT 628 and the second pixel electrode 624 connected to the TFT 628overlaps with the counter substrate 601, and liquid crystals areinjected therebetween. The counter substrate 601 is provided with thelight shielding film 632, the color film 636, the planarizing film 637,and the like. Since the pixel electrode is provided on the substrate 600side, the pixel electrode is not provided on the counter substrate 601side. The liquid crystal layer 650 is formed between the substrate 600and the counter substrate 601.

A first pixel electrode 607, a capacitor wiring 604 connected to thefirst pixel electrode 607, and the TFT 628 described in Embodiment Mode1 are formed over the substrate 600. The first pixel electrode 607 canbe formed of a material similar to the pixel electrode 77 described inEmbodiment Mode 1. Further, the first pixel electrode 607 is formed in ashape which is compartmentalized roughly in a pixel shape. Note that thegate insulating film 606 is formed over the first pixel electrode 607and the capacitor wiring 604.

The wirings 616 and 618 of the TFT 628 are formed over the gateinsulating film 606. The wiring 616 serves as a data line extending inone direction, through which a video signal is transmitted in a liquidcrystal display panel, and is connected to a source region of the TFT628 and serves as one of a source electrode and a drain electrode. Thewiring 618 serves as the other of the source electrode and the drainelectrode, and is connected to the second pixel electrode 624.

The insulating film 620 is formed over the wirings 616 and 618. Further,over the insulating film 620, the second pixel electrode 624 connectedto the wiring 618 in a contact hole formed in the insulating film 620 isformed. The second pixel electrode 624 is formed of a material similarto the pixel electrode 77 described in Embodiment Mode 1.

In this manner, the TFT 628 and the second pixel electrode 624 connectedto the TFT 628 are formed over the substrate 600. Note that a storagecapacitor is formed between the first pixel electrode 607 and the secondpixel electrode 624.

FIG. 26 is a plan view illustrating a structure of the pixel electrode.FIG. 25 is a cross-sectional structure corresponding to dashed line O-Pin FIG. 26. The slit 625 is provided for the second pixel electrode 624.The slit 625 is for controlling alignment of liquid crystals. In thiscase, an electric field is generated between the first pixel electrode607 and the second pixel electrode 624. The thickness of the gateinsulating film 606 formed between the first pixel electrode 607 and thesecond pixel electrode 624 is 50 to 200 nm, which is thin enoughcompared with the liquid crystal layer with a thickness of 2 to 10 μm.Therefore, an electric field is generated substantially in parallel (ina horizontal direction) to the substrate 600. Alignment of liquidcrystals is controlled by the electric field. Liquid crystal moleculesare horizontally rotated with the use of this electric field in adirection almost parallel to the substrate. In this case, since theliquid crystal molecules are horizontal in any state, there are feweffects of contrast or the like depending on the angle in viewing; thus,the viewing angle is expanded. Further, since both the first pixelelectrode 607 and the second pixel electrode 624 are light-transmittingelectrodes, aperture ratio can be improved.

Next, another example of a horizontal electric-field liquid crystaldisplay device will be described.

FIGS. 27 and 28 each show a pixel structure of an IPS mode liquidcrystal display device. FIG. 28 is a plan view, and FIG. 27 shows across-sectional structure taken along dashed line I-J in FIG. 28. Thefollowing explanation will be made with reference to both the drawings.

FIG. 27 shows a state in which the substrate 600 provided with the TFT628 and the second pixel electrode 624 connected to the TFT 628 overlapswith the counter substrate 601, and liquid crystals are injectedtherebetween. The counter substrate 601 is provided with the lightshielding film 632, the color film 636, the planarizing film 637, andthe like. Since the pixel electrode is provided on the substrate 600side, the pixel electrode is not provided on the counter substrate 601side. The liquid crystal layer 650 is formed between the substrate 600and the counter substrate 601.

A common potential line 609 and the TFT 628 described in Embodiment Mode1 are formed over the substrate 600. The common potential line 609 canbe formed at the same time as the gate wiring 602 of the TFT 628.Further, the common potential line 609 is formed in a shape which iscompartmentalized roughly in a pixel shape.

The wirings 616 and 618 of the TFT 628 are formed over the gateinsulating film 606. The wiring 616 serves as a data line extending inone direction, through which a video signal is transmitted in a liquidcrystal display panel, and is connected to a source region of the TFT628 and serves as one of a source electrode and a drain electrode. Thewiring 618 serves as the other of the source electrode and the drainelectrode, and is connected to the second pixel electrode 624.

The insulating film 620 is formed over the wirings 616 and 618. Further,over the insulating film 620, the second pixel electrode 624 connectedto the wiring 618 in the contact hole 623 formed in the insulating film620 is formed. The second pixel electrode 624 is formed of a materialsimilar to the pixel electrode 77 described in Embodiment Mode 1. Notethat, as shown in FIG. 28, the second pixel electrode 624 is formed soas to generate a horizontal electric field with a comb-shaped electrodewhich is formed at the same time as the common potential line 609.Further, a comb-like portion of the second pixel electrode 624 and thecomb-like electrode that is formed at the same time as the commonpotential line 609 are formed so as to alternately engage with eachother.

When an electric field is generated between the potential applied to thesecond pixel electrode 624 and the potential of the common potentialline 609, the alignment of liquid crystals is controlled by thiselectric field. Liquid crystal molecules are horizontally rotated withthe use of this electric field in a direction almost parallel to thissubstrate. In this case, since the liquid crystal molecules arehorizontal in any state, there are few effects of contrast or the likedepending on the angle in viewing; thus, the viewing angle is expanded.

In such a manner, the TFT 628 and the second pixel electrode 624connected to the TFT 628 are formed over the substrate 600. A storagecapacitor is formed by provision of the gate insulating film 606 betweenthe common potential line 609 and a capacitor electrode 615. Thecapacitor electrode 615 and the second pixel electrode 624 are connectedin the contact hole 623.

Next, an IPS mode liquid crystal display device will be described.

FIGS. 29 and 30 each show a pixel structure of an IPS mode liquidcrystal display device. FIG. 30 is a plan view. FIG. 29 shows across-sectional structure along dashed line K-L in FIG. 30. Thefollowing explanation will be made with reference to both the drawings.

The second pixel electrode 624 is connected to the TFT 628 through thewiring 618 in the contact hole 623. The wiring 616 which serves as adata line is connected to the TFT 628. As the TFT 628, any of the TFTsdescribed in Embodiment Mode 1 can be used.

The second pixel electrode 624 is formed of the pixel electrode 77described in Embodiment Mode 1.

The counter substrate 601 is provided with the light shielding film 632,the color film 636, and the counter electrode 640. Further, theplanarizing film 637 is formed between the second colored film 636 andthe counter electrode 640 to prevent alignment disorder of liquidcrystals. The liquid crystal layer 650 is formed between the secondpixel electrode 624 and the counter electrode 640, with the alignmentfilms 648 and 649 interposed therebetween.

A liquid crystal element is formed by overlapping the second pixelelectrode 624, the liquid crystal layer 650, and the counter electrode640 with each other.

The substrate 600 or the counter substrate 601 may be provided with acolor filter, a shielding film (a black matrix) for preventingdisclination, or the like. Further, a polarizing plate is attached to asurface of the substrate 600, which is opposite to a surface providedfor the thin film transistor. Moreover, a polarizing plate is attachedto a surface of the counter substrate 601, which is opposite to asurface provided for the counter electrode 640.

Through the above-described steps, a liquid crystal display device canbe manufactured as a display device. Since a thin film transistor withless off current and with high electric properties and high reliabilityis used in the liquid crystal display device of this embodiment mode,the liquid crystal display device has high contrast and high visibility.

(Embodiment Mode 8)

Next, a structure of a display panel, which is one mode of the displaydevice of the present invention, will be described below. A liquidcrystal display panel which is one mode of a liquid crystal displaydevice having a liquid crystal element as a display element (the liquidcrystal display panel is also referred to as a liquid crystal panel) anda light-emitting display panel which is one mode of a display devicehaving a light-emitting element as a display element (the light-emittingdisplay panel is also referred to as a light-emitting panel) will bedescribed in this embodiment mode.

FIG. 9A shows a mode of a light-emitting display panel in which a signalline driver circuit 6013 which is separately formed is connected to apixel portion 6012 formed over a substrate 6011. The pixel portion 6012and a scanning line driver circuit 6014 are each formed of thin filmtransistors including a microcrystalline semiconductor film. By formingthe signal line driver circuit of a transistor by which higher mobilitycan be obtained compared to the thin film transistor including amicrocrystalline semiconductor film, operation of the signal line drivercircuit, which demands a higher driving frequency than the scanning linedriver circuit, can be stabilized. Note that the signal line drivercircuit 6013 may be formed of a transistor including asingle-crystalline semiconductor, a thin film transistor including apolycrystalline semiconductor, or a transistor including an SOL Thepixel portion 6012, the signal line driver circuit 6013, and thescanning line driver circuit 6014 are each supplied with a potential ofa power supply, a variety of signals, and the like via an FPC 6015.

Note that the signal driver circuit and the scanning line driver circuitmay be formed over the same substrate as the pixel portion.

Further, when a driver circuit is separately formed, a substrateprovided with the driver circuit is not necessarily attached to asubstrate provided with a pixel portion, and may be attached to, forexample, an FPC. A mode of a display panel in which a signal line drivercircuit 6023 is formed separately and is connected to a pixel portion6022 and a scanning line driver circuit 6024 that are formed over asubstrate 6021 is shown in FIG. 9B. The pixel portion 6022 and thescanning line driver circuit 6024 are each formed of thin filmtransistors including a microcrystalline semiconductor film. The signalline driver circuit 6023 is connected to the pixel portion 6022 via anFPC 6025. The pixel portion 6022, the signal line driver circuit 6023,and the scanning line driver circuit 6024 are each supplied with apotential of a power source, a variety of signals, and the like via theFPC 6025.

Alternatively, only part of a signal line driver circuit or part of ascanning line driver circuit may be formed over the same substrate as apixel portion by using a thin film transistor using a microcrystallinesemiconductor film, and the other part of the driver circuit may beseparately formed and electrically connected to the pixel portion. Amode of a display panel in which an analog switch 6033 a included in asignal driver circuit is formed over the same substrate 6031 as a pixelportion 6032 and a scanning line driver circuit 6034, and a shiftregister 6033 b included in the signal line driver circuit is formedseparately over a different substrate and then attached to the substrate6031 is shown in FIG. 9C. The pixel portion 6032 and the scanning linedriver circuit 6034 are each formed of thin film transistors including amicrocrystalline semiconductor film. The shift resistor 6033 b includedin the signal line driver circuit is connected to the pixel portion 6032via an FPC 6035. The pixel portion 6032, the signal line driver circuit,and the scanning line driver circuit 6034 are each supplied with apotential of a power supply, a variety of signals, and the like via theFPC 6035.

As shown in FIGS. 9A to 9C, in the display device of the presentinvention, an entire driver circuit or part thereof can be formed overthe same substrate as a pixel portion, of thin film transistorsincluding a microcrystalline semiconductor film.

Note that there is no particular limitation on a connection method ofthe substrate formed separately; a COG method, a wire bonding method, aTAB method, or the like can be used. Further, a connection position isnot limited to the positions shown in FIGS. 9A to 9C as long aselectrical connection is possible. Moreover, a controller, a CPU, amemory, or the like may be formed separately and connected.

Note that a signal line driver circuit used in the present invention isnot limited to a mode including only a shift resistor and an analogswitch. In addition to the shift register and the analog switch, anothercircuit such as a buffer, a level shifter, or a source follower may beincluded. Further, the shift resistor and the analog switch are notnecessarily provided; for example, a different circuit such as a decodercircuit by which selection of a signal line is possible may be usedinstead of the shift resistor, and a latch or the like may be usedinstead of the analog switch.

Next, an external view and a cross section of a light-emitting displaypanel, which is one mode of the display device of the present invention,will be described with reference to FIGS. 12A and 12B. FIG. 12A is a topview of a panel in which a thin film transistor including amicrocrystalline semiconductor film and a light-emitting element whichare formed over a first substrate are sealed between the first substrateand a second substrate with a sealing material. FIG. 12B is across-sectional view along line E-F in FIG. 12A.

The sealing material 4505 is provided so as to surround a pixel portion4502 and a scanning line driver circuit 4504 which are provided over thefirst substrate 4501. Further, the second substrate 4506 is providedover the pixel portion 4502 and the scanning line driver circuit 4504.Therefore, the pixel portion 4502 and the scanning line driver circuit4504 are sealed together with a filler 4507 by the first substrate 4501,the sealing material 4505, and the second substrate 4506. Further, asignal line driver circuit 4503 formed using a polycrystallinesemiconductor film over a different substrate is mounted on a regionover the first substrate 4501, which is different from the regionsurrounded by the sealing material 4505. Note that, although an examplein which the signal line driver circuit including thin film transistorsusing a polycrystalline semiconductor film is attached to the firstsubstrate 4501 is described in this embodiment mode, a signal linedriver circuit may be formed using a transistor including a singlecrystalline semiconductor and attached to a substrate. FIG. 12Billustrates a thin film transistor 4509 formed using a polycrystallinesemiconductor film, which is included in the signal line driver circuit4503.

Further, each of the pixel portion 4502 and the scanning line drivercircuit 4504 which are provided over the first substrate 4501 includes aplurality of thin film transistors. FIG. 12B illustrates thin filmtransistors 4510 and 4520 included in the pixel portion 4502. Note that,although description is made on the assumption that the thin filmtransistor 4510 is a driving TFT in this embodiment mode, the thin filmtransistor 4510 may be a TFT for current control or a TFT for erasingdata. The thin film transistors 4510 and 4520 are thin film transistorseach using a microcrystalline semiconductor film, and to each of whichany thin film transistor described in Embodiment Mode 1, 2, or 4 can beapplied and a similar manufacturing process to any of Embodiment Modes 1to 5 can be employed. In this embodiment mode, the thin film transistor4510 is an n-channel thin film transistor using, as a source and drainregions, a semiconductor film to which an impurity which imparts n-typeconductivity is added, and the thin film transistor 4520 is a p-channelthin film transistor using, as a source and drain regions, asemiconductor film to which an impurity which imparts p-typeconductivity is added. The thin film transistor in the present inventioncan be either an n-channel thin film transistor or a p-channel thin filmtransistor, and a CMOS (complementary metal oxide semiconductor) whichincludes an n-channel thin film transistor and a p-channel thin filmtransistor can be provided in a display device.

Further, reference numeral 4511 denotes a light-emitting element. Apixel electrode included in the light-emitting element 4511 iselectrically connected to a source or drain electrode of the thin filmtransistor 4510 through a wiring 4517. Further, in this embodiment mode,a common electrode of the light-emitting element 4511 and a transparentconductive film 4512 which is made of a light-transmitting conductivematerial are electrically connected to each other. Note that thestructure of the light-emitting element 4511 is not limited to thestructure described in this embodiment mode. The structure of thelight-emitting element 4511 can be changed as appropriate depending on adirection in which light is extracted from the light-emitting element4511, the conductivity type of the thin film transistor 4510, or thelike.

Further, a variety of signals and a potential which are applied to thesignal line driver circuit 4503 that is formed separately, the scanningline driver circuit 4504, or the pixel portion 4502 are supplied from anFPC 4518 through a wiring 4514 and a wiring 4515, although not shown inthe cross-sectional view of FIG. 12B.

In this embodiment mode, a connection terminal 4516 is formed of thesame conductive film as the pixel electrode included in thelight-emitting element 4511. Further, the wirings 4514 and 4515 areformed of the same conductive film as the wiring 4517.

The connection terminal 4516 is electrically connected to a terminalincluded in the FPC 4518 through an anisotropic conductive film 4519.

The substrate located in the direction in which light is extracted fromthe light-emitting element 4511 needs to be transparent. In this case, alight-transmitting material such as a glass plate, a plastic plate, apolyester film, or an acrylic film is used.

Further, as the filler 4507, an ultraviolet curable resin or athermosetting resin can be used, as well as an inert gas such asnitrogen or argon. For example, PVC (polyvinyl chloride), acrylic,polyimide, an epoxy resin, a silicone resin, PVB (polyvinyl butyral), orEVA (ethylene vinyl acetate) can be used. In this embodiment mode,nitrogen is used for the filler 4507.

Further, if needed, an optical film such as a polarizing plate, acircular polarizing plate (including an elliptical polarizing plate), aretardation plate (a quarter-wave plate or a half-wave plate), or acolor filter may be provided for an emission surface of thelight-emitting element as appropriate. Further, the polarizing plate orthe circularly polarizing plate may be provided with an anti-reflectionfilm. For example, an anti-glare treatment which can diffuse reflectedlight with roughness of the surface, and reduce reflection can beperformed.

Note that, although an example in which the signal line driver circuit4503 is formed separately and mounted on the first substrate 4501 isshown in FIGS. 12A and 1213, this embodiment mode is not limited to thisstructure. A scanning line driver circuit may be separately formed andattached to a substrate, or only part of a signal line driver circuit orpart of a scanning line driver circuit may be separately formed andattached to a substrate.

Next, the appearance and a cross section of a liquid crystal displaypanel which is one mode of the liquid crystal display device of thepresent invention will be described with reference to FIGS. 16A and 16B.FIG. 16A is a top view of a panel in which a thin film transistor 4010including a microcrystalline semiconductor film and a liquid crystalelement 4013 which are formed over a first substrate 4001 are sealedbetween the first substrate 4001 and a second substrate 4006 with asealant 4005, and FIG. 16B is a cross-sectional view taken along lineM-N in FIG. 16A.

The sealant 4005 is provided so as to surround the pixel portion 4002and the scanning line driver circuit 4004 which are provided over thefirst substrate 4001. Further, the second substrate 4006 is providedover the pixel portion 4002 and the scanning line driver circuit 4004.Therefore, the pixel portion 4002 and the scanning line driver circuit4004 are sealed together with liquid crystals 4008 by the firstsubstrate 4001, the sealing material 4005, and the second substrate4006. Further, a signal line driver circuit 4003 that is fowled using apolycrystalline semiconductor film over a separately prepared substrateis mounted at a region that is different from the region surrounded bythe sealant 4005 over the first substrate 4001. Note that, although anexample in which the signal line driver circuit 4003 including thin filmtransistors using a polycrystalline semiconductor film is attached tothe first substrate 4001 is described in this embodiment mode, a signalline driver circuit may be formed of transistors each using asingle-crystalline semiconductor film and attached to the firstsubstrate 4001. FIG. 16B illustrates a thin film transistor 4009 that isformed using a polycrystalline semiconductor film and included in thesignal line driver circuit 4003.

Further, the pixel portion 4002 and the scanning line driver circuit4004 provided over the first substrate 4001 each include a plurality ofthin film transistors, and the thin film transistor 4010 included in thepixel portion 4002 is illustrated in FIG. 1613. The thin film transistor4010 is a thin film transistor using a microcrystalline semiconductorfilm, and to which any thin film transistor described in Embodiment Mode1, 2, or 4 can be applied and a similar manufacturing process to any ofEmbodiment Modes 1 to 5 can be employed.

Further, reference numeral 4013 denotes a liquid crystal element, and apixel electrode 4030 included in the liquid crystal element 4013 iselectrically connected to the thin film transistor 4010 via a wiring4040. A counter electrode 4031 of the liquid crystal element 4013 isprovided for the second substrate 4006. A region where the pixelelectrode 4030, the counter electrode 4031, and the liquid crystals 4008overlap with each other corresponds to the liquid crystal element 4013.

Note that, for each of the first substrate 4001 and the second substrate4006, glass, metal (typically, stainless steel), ceramic, or plastic canbe used. As for the plastic, an FRP (fiberglass-reinforced plastics)plate, a PVF (polyvinyl fluoride) film, a polyester film, or an acrylicresin film can be used. Further, a sheet with a structure in which analuminum foil is sandwiched with a PVF film or a polyester film can beused as well.

Further, reference numeral 4035 denotes a spherical spacer and isprovided to control a distance (a cell gap) between the pixel electrode4030 and the counter electrode 4031. Note that a spacer which isobtained by selectively etching an insulating film may be used as well.

Further, a variety of signals and a potential supplied to the separatelyformed signal line driver circuit 4003, the scanning line driver circuit4004, and the pixel portion 4002 are supplied from an FPC 4018 throughlead wirings 4014 and 4015.

In this embodiment mode, a connection terminal 4016 is formed of thesame conductive film as the pixel electrode 4030 included in the liquidcrystal element 4013. In addition, the lead wirings 4014 and 4015 areformed of the same conductive film as the wiring 4040.

The connection terminal 4016 is electrically connected to a terminalincluded in the FPC 4018 through an anisotropic conductive film 4019.

Note that, although not shown, the liquid crystal display devicedescribed in this embodiment mode includes an alignment film and apolarizing plate, and further, may include a color filter and/or ashielding film.

Note that, although an example in which the signal line driver circuit4003 is formed separately and mounted on the first substrate 4001 isshown in FIGS. 16A and 16B, this embodiment mode is not limited to thisstructure. A scanning line driver circuit may be separately formed andattached to a substrate, or only part of a signal line driver circuit orpart of a scanning line driver circuit may be separately formed andattached to a substrate.

This embodiment mode can be implemented in combination with any of thestructures described in the other embodiment modes, as appropriate.

(Embodiment Mode 9)

The display device or the like obtained by the present invention can beused for a display module (an active matrix EL module or a liquidcrystal module). That is, the present invention can be implemented inall electronic devices in which such a module is incorporated into adisplay portion.

As such electronic devices, cameras such as video cameras and digitalcameras; displays that can be mounted on a head (goggle-type displays);car navigation systems; projectors; car stereos; personal computers;portable information terminals (e.g., mobile computers, mobile phones,and electronic books); and the like can be given. Examples of thesedevices are illustrated in FIGS. 7A to 7D.

FIG. 7A shows a television device. The television device can becompleted by incorporating a display module into a chassis as shown inFIG. 7A. A display panel provided with components up to an FPC is alsocalled a display module. A main screen 2003 is formed by using thedisplay module, and a speaker portion 2009, an operation switch, and thelike are provided as other attached structures. In this manner, thetelevision device can be completed.

As shown in FIG. 7A, a display panel 2002 using a display element isincorporated into a chassis 2001. The television device can receivegeneral TV broadcast by a receiver 2005, and can be connected to a wiredor wireless communication network via a modem 2004 so that one-way (froma sender to a receiver) or two-way (between a sender and a receiver orbetween receivers) information communication can be performed. Thetelevision device can be operated by switches incorporated in thechassis or by a remote controller 2006 separated from the main body. Adisplay portion 2007 that displays information to be outputted may alsobe provided in this remote controller.

Further, the television device may include a sub screen 2008 formedusing a second display panel to display channels, volume, and the like,in addition to the main screen 2003. In this structure, the main screen2003 may be formed of a light-emitting display panel which has anexcellent viewing angle, and the sub-screen 2008 may be formed of aliquid crystal display panel by which display can be performed at lowpower consumption. Alternatively, when reduction in power consumption isprioritized, a structure may be employed in which the main screen 2003is formed of a liquid crystal display panel, the sub screen 2008 isformed of a light-emitting display panel, and the sub screen 2008 can beturned on and off,

FIG. 8 is a block diagram of a main structure of a television device. Apixel portion 901 is formed in a display panel. A signal line drivercircuit 902 and a scanning line driver circuit 903 may be mounted on thelight-emitting display panel by a COG method.

As a structure of another external circuit, a video signal amplifiercircuit 905 that amplifies a video signal among signals received by atuner 904, a video signal processing circuit 906 that converts thesignal outputted from the video signal amplifier circuit 905 into achrominance signal corresponding to each color of red, green, and blue,a control circuit 907 that converts the video signal into an inputspecification of a driver IC, and the like are provided on a videosignal input side. The control circuit 907 outputs signals to both ascanning line side and a signal line side. In the case of digitaldriving, a signal dividing circuit 908 may be provided on the signalline side so that an input digital signal is divided into m pieces to besupplied.

Among signals received by the tuner 904, an audio signal is transmittedto an audio signal amplifier circuit 909, and an output of the audiosignal amplifier circuit 909 is supplied through an audio signalprocessing circuit 910 to a speaker 913. A control circuit 911 receivescontrol information of a receiving station (reception frequency) orsound volume from an input portion 912 and transmits signals to thetuner 904 and the audio signal processing circuit 910.

It is needless to say that the present invention is not limited to atelevision device and the present invention can be used for variousapplications as a large display medium such as an information displayboard at train stations, airports, and the like, and an advertisementboard on street as well as a monitor of a personal computer.

FIG. 7B shows an example of a mobile phone 2301. The mobile phone 2301includes a display portion 2302, an operation portion 2303, and thelike. When the display device described in the above-describedembodiment mode is applied to the display portion 2302, reliability andmass productivity of the mobile phone 2301 can be improved.

A mobile computer shown in FIG. 7C includes a main body 2401, a displayportion 2402, and the like. When the display device described in theabove-described embodiment mode is applied to the display portion 2402,reliability and mass productivity of the mobile computer can beimproved.

FIG. 7D shows a desk lamp including a lighting portion 2501, a shade2502, an adjustable arm 2503, a support 2504, a base 2505, and a powersupply switch 2506. The desk lamp can be manufactured by applying thedisplay device of the present invention to the lighting portion 2501.Note that the light includes, in its category, ceiling lights, walllights, and the like. By applying the display device described in theabove-described embodiment mode, reliability and mass productivity ofthe desk lamp can be improved.

This application is based on Japanese Patent Application Serial No.2007-227073 filed with Japan Patent Office on Aug. 31, 2007, the entirecontents of which are hereby incorporated by reference.

1. A manufacturing method of a display device, comprising the steps of:forming a gate electrode over a substrate; forming a gate insulatingfilm over the gate electrode; forming a microcrystalline semiconductorfilm over the gate insulating film, the microcrystalline semiconductorfilm including a channel formation region; forming a channel protectivelayer over the channel formation region; forming a buffer layer over thechannel protective layer; forming a source and drain regions over thebuffer layer; forming a source and drain electrodes over the source anddrain regions; and forming an impurity region by adding an impurityelement which imparts one conductivity type selectively in the channelformation region of the microcrystalline semiconductor film, through thechannel protective layer with a use of the source and drain electrodesas masks.
 2. The manufacturing method of a display device according toclaim 1, after forming the impurity region, further comprising a step ofirradiating the impurity region with a laser light through the channelprotective layer.
 3. The manufacturing method of a display deviceaccording to claim 1, wherein the buffer layer is formed of an amorphoussemiconductor film.
 4. The manufacturing method of a display deviceaccording to claim 1, wherein the impurity region is formed in thechannel formation region of the microcrystalline semiconductor filmbetween the source and drain electrodes.
 5. The manufacturing method ofa display device according to claim 1, wherein the impurity elementwhich imparts one conductivity type is an impurity element which impartsp-type conductivity.
 6. A manufacturing method of a display device,comprising the steps of: forming a gate electrode over a substrate;forming a gate insulating film over the gate electrode; forming amicrocrystalline semiconductor film over the gate insulating film, themicrocrystalline semiconductor film including a channel formationregion; forming a channel protective layer over the channel formationregion; forming a buffer layer over the channel protective layer;forming a source and drain regions over the buffer layer; forming asource and drain electrodes over the source and drain regions; formingan impurity region by adding an impurity element which imparts oneconductivity type selectively in the channel formation region of themicrocrystalline semiconductor film, through the channel protectivelayer with a use of the source and drain electrodes as masks; andforming a pixel electrode which is electrically connected to one of thesource and drain electrodes.
 7. The manufacturing method of a displaydevice according to claim 6, after forming the impurity region, furthercomprising a step of irradiating the impurity region with a laser lightthrough the channel protective layer.
 8. The manufacturing method of adisplay device according to claim 6, wherein the buffer layer is formedof an amorphous semiconductor film.
 9. The manufacturing method of adisplay device according to claim 6, wherein the impurity region isformed in the channel formation region of the microcrystallinesemiconductor film between the source and drain electrodes.
 10. Themanufacturing method of a display device according to claim 6, whereinthe impurity element which imparts one conductivity type is an impurityelement which imparts p-type conductivity.